TY - JOUR
AU - Mensink, Paul, J
AB - Abstract Porbeagles throughout the North Atlantic have experienced severe population decline through overfishing, with the northeastern population listed as critically endangered. Management of this population is constrained by the paucity of data on porbeagle population structure, distribution and behaviour in this region. Here we use a long-term (47 year) Irish capture-mark-recapture dataset to investigate the population structure, spatial distribution and seasonal movements of this species. From 1970–2017, a total of 268 sharks (9 recaptures) were ID tagged, with most individuals likely being juvenile based on length at maturity estimates (mean total length = 143.9 cm, SD = 35.4). Almost all captures were recorded at three distinct locations near angling hubs along the south, west and north coasts with catches peaking in August. Long-term trends in capture date indicated a shift towards earlier capture dates in the northern site (n = 153). Our findings suggest Irish waters may act as a persistent summer aggregation site for juveniles, which show evidence for seasonal site fidelity, returning to nearby locations between years. These findings demonstrate the utility of such programmes, which can be implemented, with minimal expense by engaging with the angling sector, to elucidate the population structure and distribution of wide-ranging fish species. Introduction Many regional action plans for wide-ranging species are hindered by a lack of robust population data and basic information on seasonal patterns of residency and movement (Baum et al., 2003; Runge et al., 2014). Such gaps in our knowledge have been redressed significantly since the advent of biotelemetry, with clearly defined patterns of movement now identified for many wide-ranging species (e.g. Hays et al., 2004; Skomal et al., 2009; Domeier and Nasby-Lucas, 2013; Weimerskirch et al., 2015). However, given that animal tracking can be prohibitively expensive or logistically demanding, it is often difficult to discern trends at a population level, or over protracted time-scales, until substantial datasets can be considered meta-analytically (e.g. Sequeira et al., 2018). When attempting to fill in these gaps in space and time, there is great value in mark-recapture data that can contain information on hundreds, or thousands of individuals over many years (e.g. Hurst et al., 1999; Kohler et al., 2002; Brown and Oschadleus, 2009; Thorup et al., 2014). Beyond patterns of movement and residence, mark-recapture datasets can also provide more nuanced information on population structure, patterns of segregation and on occasion, growth of individuals (e.g. Kohler et al., 2002; Casale et al., 2009; Hamel et al., 2014). In the marine environment, the impetus for gathering movement and population data for exploited marine fishes increasingly arises from conservation concerns. For example, many shark stocks are under increasing pressure owing to overfishing (Ferretti et al., 2010; Worm et al., 2013), with 17% of shark and ray species listed as Critically Endangered, Endangered, or Vulnerable and a further 47% listed as Data Deficient by the IUCN (Lack and Sant, 2009; Polidoro et al., 2009). Many of these species are also broad-ranging, bringing them into contact with different levels of protection or exploitation throughout their movements (Techera and Klein, 2011; Campana, 2016). From this perspective, porbeagle sharks (Lamna nasas) have received increased attention following severe population declines in both the northeast and northwest Atlantic as a result of fishing pressure (DFO, 2005; Stevens et al., 2006a, b; Campana et al., 2013; NOAA, 2013; Curtis et al., 2016). This species is widely distributed and can be found in cold-temperate waters across the globe, with subpopulations in the northeast and northwest Atlantic and a distinct population in the southern hemisphere (Stevens et al., 2006a). The species is currently listed by the IUCN as “Vulnerable” throughout its range and “Critically Endangered” in the northeast Atlantic (Stevens et al., 2006a, b; Nieto et al., 2015). For the northeast Atlantic population, satellite tags have revealed a pattern of variable and wide-ranging movement amongst individuals that is broadly consistent with the classic migratory paradigm of movement between higher and lower latitudes in summer and winter (Biais et al., 2017). However, it can be challenging to extrapolate from these typically short-term, fine-scale annual movements of individuals to the long-term patterns of seasonal occurrence for the wider population that are critical components of regional management plans (Runge et al., 2014). Complementing individual movements obtained by satellite tags with long-term mark-recapture data could help elucidate the underlying population trends for this critically endangered population. Given the status of this population, analysis of mark-recapture data may also prove useful in beginning to identify specific distribution patterns, which could potentially increase the risk of overfishing, as documented in other regions (Aasen, 1963; O’Boyle et al., 1998; Pade et al., 2009). For example, O’Boyle et al. (1998) found that mature females in the northwest Atlantic segregated from the main body of the population in spring, which coincided with the parturition period for porbeagles in this subpopulation (Jensen et al., 2002). It has also been suggested that juveniles and sub-adults may have more restricted ranges and movements and might be limited to lower latitudes than larger sharks and particularly large mature females, which can attain greater sizes than males (Aasen, 1963; O’Boyle et al., 1998; Francis and Stevens, 2000; Natanson et al., 2002; Francis, 2016; Biais et al., 2017). This type of size-based segregation may be responsible for a disproportionate amount of fishery-related mortality of juveniles in coastal New Zealand waters (Semba et al., 2013; Francis, 2016), highlighting the direct relevance of this behaviour to management outcomes for porbeagles. Satellite tracking by Biais et al. (2017) indicated that both sex- and size-based spatial segregation could be present in the northeast Atlantic population and mark-recapture data could help provide further empirical evidence to test this hypothesis. In this study, we used nearly five decades of mark-recapture records from the National Marine Sport Fish Tagging (MSFT) programme database, which is managed by Inland Fisheries Ireland (IFI), the state agency responsible for the protection, management and conservation of Ireland’s inland fisheries and sea angling resources. These data were utilized to examine long-term trends in porbeagle population structure, distribution, and phenology (i.e. date of capture). This region lies at a relatively low latitude for the species and is close to the presumed overwintering grounds for this population (Pade et al., 2009; Biais et al., 2017). We therefore hypothesised that the population would be biased toward juveniles given their potentially more restricted movements and constraint to lower latitudes. Methods Data collection Between 1970 and 2017 a total of 276 porbeagle tagging events were collated by IFI, with the majority of sharks (94%) captured between June and October, during the shark angling season. In total 268 of these sharks were tagged by charter boat skippers or recreational anglers from the Irish coast, from a latitude of ∼51.2° to 55.6°, covering a latitudinal range of 4.4°. Almost all (266) captures were clustered into three geographic regions based on their proximity to the nearest angling hub port(s); southern—coastal waters around Kinsale, western—coastal waters near Galway Bay, northern-coastal waters around Malin Head, with two captures recorded from the northeast coast (Figure 1). In addition, a further eight sharks were recorded from coastal waters near the Isle of Wight off the English coast by an English charter boat. Mean sea surface temperature (SST) was 13.9°C (SD 1.3) when porbeagle was captured by anglers around Malin Head between 2002 and 2015 (McVeigh, pers. comm.). Upon capture, all sharks were tagged with jumbo tags (Dalton Lifetime Jumbotag) through the dorsal fin. The unique code of each tag was then used to identify individual sharks upon recapture. This code was considered alongside tag type (in this case “J” for jumbo) to create a seven-character identification system for all sharks (e.g. Jumbo tag number 75 would have the code “J000075”). At the time of capture, basic biometric data such as fork length (FL), total length (TL), girth, mass, and sex were recorded where possible. Skippers were requested to accurately measure and weigh captured sharks, however the possibility that some measurements were estimated cannot be ruled out. In addition, the date of capture and location from which the shark was caught were also recorded (GPS coordinates or location description). The tagged sharks were then released and the aforementioned information reported to IFI. Each tag carries the contact details for IFI’s MSFT programme, allowing any relevant information to be reported to IFI following any subsequent recapture. Where a location was given as a location description rather than coordinates (e.g. 100 yards West of Inishmaan) this was then converted to decimal GPS coordinates (decimal degrees) for mapping (n = 43). Figure 1. Open in new tabDownload slide Capture locations for tagged porbeagles in (a) northern geographic group (n = 153), (b) the western group (n = 54), and (c) the southern group (n = 59). Two fish outside of these groups, which were nominally tagged at the North East location (55.06°, −06.05°) were tagged within a 40-km radius of the reported tagging site. Figure 1. Open in new tabDownload slide Capture locations for tagged porbeagles in (a) northern geographic group (n = 153), (b) the western group (n = 54), and (c) the southern group (n = 59). Two fish outside of these groups, which were nominally tagged at the North East location (55.06°, −06.05°) were tagged within a 40-km radius of the reported tagging site. Calculation of length, mass, and age In total 173 of the 268 sharks tagged in Irish waters had some form of length measurement recorded at the point of tagging (FL and TL recorded n = 77, FL only n = 46, TL only n = 50) with 95 having no form of length measurement. Where length data were missing, TL was calculated from FL and vice versa using the formula from Kohler et al. (1996): FL=0.8971TL+1.7939 No length at maturity estimates currently exist for the northeast Atlantic porbeagle subpopulation; therefore, we estimated maturity status of individuals in our dataset from length at maturity data collected by Jensen et al. (2002) for the northwest Atlantic population (females = 382, males = 393). In Jensen et al. (2002), FL at maturity ranged from 210 to 230 cm in females (50% mature at 218 cm) and 162–185 cm in males (50% mature at 174 cm). For our study, maturity status was assigned using Jensen’s minimum FL at maturity. In cases where FL exceeded the minimum length of maturity (n = 8), the FL was then compared with the FL at which 50% of individuals have reached maturity. Where FL was unrecorded the converted value calculated from TL was used. Where sex was not recorded FL was compared with both the minimum FL at maturity and 50% maturity figure for both males and females. Ages of captured sharks were estimated using the Von Bertalanffy (VB) growth parameters (following Natanson et al., 2002) for the northwest Atlantic. For individuals where sex was unknown the combined VB growth parameters were used. Shark ages were calculated based on TL using the “age slicing” function within the package ALKr in Rstudio (Loff et al., 2014; R Core Team, 2017). This gives an estimation of a population’s age distribution from its length distribution based on its VB growth curve, as described by Kell and Kell (2011). The age distribution is calculated by using the inverse of the VB growth curve, i.e. Age=t0- log⁡(1-length)/K Where K is the growth rate and t0 the mean length at time equals zero i.e. at birth Statistical analysis We used Bayesian regressions to explore how long-term annual trends in capture date (mean ordinal day of the year) varied as a function of both geographic region (southern, western, and northern) and SST. Annual trends in capture date were modelled as a second-order random walk (sigma = 2, precision = 0.01). We were particularly interested in determining if there was a common temporal trend in capture date amongst all geographic regions or if each region was best characterised by its own temporal trend. To test between these two alternatives, we used deviance information criterion (DIC) to compare a model with a joint temporal trend for all regions vs. a model with individual trends for each region over time (Zuur et al., 2017). Movement and distribution in elasmobranchs can be strongly correlated with SST (Hopkins and Cech, 2003; Robbins, 2007; Mitchell et al., 2014), and porbeagle distribution has shown links to water temperature, albeit measured at depth rather than at the sea surface, previously (Campana and Joyce, 2004), suggesting water temperature could be an important determinant of their movement. To examine the effect of SST on variation in annual capture date, we analysed data from the northern region only. The large number of overall captures in this region (n = 153, 2006–2017) and the greater number of captures within each year facilitated a more robust test of SST influence on capture date. Furthermore, the northern region was much closer than the western and southern sites to the area where SST had been recorded by Met Éireann over the duration of the study (Malin Head station, 55°22′20″ N, 7°20′20″ W). We extracted the mean annual trend in capture date from the initial model for the northern site and determined how that varied as a function of mean annual SST from the Malin Head station (predictor variable). We employed an errors-in-variables regression approach to account for uncertainty around both the response variable (credible intervals around the trendline) and SST (standard deviation of annual SST). All statistical analyses were performed using R Core Team (2017). Long-term annual trends in capture date were modelled using the INLA package, which uses integrated nested Laplace approximations to estimate the posterior distribution (Rue et al., 2009). The relationship between annual mean capture date and SST was estimated using the brms package (Bürkner, 2017), which uses Markov-chain Monte Carlo simulations to sample the posterior distribution (3 chains, 5000 iterations). Results Population structure and biometric information Of the 268 sharks tagged in Irish waters, 181 individuals were sexed, with a total of 88 males and 93 females giving an approximate 1:1 sex ratio. Where TL was recorded or calculated from available FL measurements it ranged from 46 to 270 cm (mean = 143.9 cm, SD = 35.4, n = 157; see Figure 2 for actual TL values only). The TL to FL conversion was validated by comparing calculated FL values to recorded values for those sharks where both length metrics were recorded (mean difference = 8.67 cm, SD = 19.25). Mass ranged from 9.1 to 133.0 kg (mean = 35.2 kg, SD = 25.1, n = 138). Figure 2. Open in new tabDownload slide (a) Total length frequency data for tagged and measured porbeagles in Irish waters (n = 127), excluding values calculated from fork length (b) Estimated age data for sexed porbeagles tagged in Irish waters based upon VB growth curves using both recorded and calculated total length data (n = 132). Total n tagged = 268. Figure 2. Open in new tabDownload slide (a) Total length frequency data for tagged and measured porbeagles in Irish waters (n = 127), excluding values calculated from fork length (b) Estimated age data for sexed porbeagles tagged in Irish waters based upon VB growth curves using both recorded and calculated total length data (n = 132). Total n tagged = 268. Maturity status The vast majority of individuals from Irish waters was likely immature at the time of first capture. Only six males had exceeded the minimum FL at maturity (162 cm), as recorded by Jensen et al. (2002). Of these, two individuals had also surpassed both the FL at which 50% of males are understood to be mature and the FL at which all males are thought to be mature. Two females had also surpassed the minimum length at maturity (210-cm FL) with neither of these exceeding the maximum recorded length for immature females and only one exceeding the length at which 50% of females are mature. This pattern indicates that between two and six males and zero and two females from this sample were mature, with the vast majority of sexed fish being immature. Of unsexed fish, none had reached the minimum FL at maturity for males indicating that all were immature, hence an estimated 95–99% of the 173 sharks for which FL was measured or could be estimated from TL were likely immature. Estimated fish ages calculated from VB growth parameters indicate that the majority (74%) of sharks were six years old or less, with a maximum age of 14 in males and 16 in females. Estimated mean age was 4.9 years in males (SD = 3.27) and 5.7 years in females (SD = 3.96). Spatial and temporal distribution For annual trends in capture date, the model with individual trends for each region outperformed the model with a common trend for all regions (Δ DIC = 107.68). Mean date of capture occurred successively later (higher ordinal day) for the three regions according to their relative latitudinal position; however, differences between capture dates were extremely small and credible intervals for each region overlapped heavily (βSouth = 223, 95% credible intervals = 174, 253; βwest = 231, 95% credible intervals = 189, 268; βNorth = 233, 95% credible intervals = 193, 272; Figure 3). Mean ordinal days represent 11, 19, and 21 August, for the south, west, and north geographic groups, respectively. At the northern site, mean annual capture date showed a steady shift to occurring earlier in the season (Figure 3); however, this was uncorrelated with mean annual SST (β = −2.24, 95% credible intervals = −15.12, 14.00). Figure 3. Open in new tabDownload slide Long-term annual trends in capture date (ordinal day) of porbeagle sharks in Irish coastal waters from 1970 to 2013 across three different regions. Separate trends for each region were modelled with a second-order random walk process (black lines) and shaded bands around the line represent 95% credible intervals around the mean. Grey points represent individual capture dates with darker colours indicating overlapping points. Figure 3. Open in new tabDownload slide Long-term annual trends in capture date (ordinal day) of porbeagle sharks in Irish coastal waters from 1970 to 2013 across three different regions. Separate trends for each region were modelled with a second-order random walk process (black lines) and shaded bands around the line represent 95% credible intervals around the mean. Grey points represent individual capture dates with darker colours indicating overlapping points. Recaptures Nine individuals were subsequently recaptured between 1970 and 2018, a recapture rate of 3.4%. Time at liberty ranged from 71 to 3946 days (mean = 1456, SD = 1643.3) (Table 1). Five individuals were recaptured from Irish waters with the remaining individuals coming from Scotland, the Faroes, and Canada (Figure 4). The FLs and calculated FLs (Table 1) indicate that at least five individuals were most likely still immature when recaptured based on the length at maturity data from Jensen et al. (2002). No biometric information was recorded for shark J026819 upon recapture; however based on the VB growth parameters for this species calculated by Kohler et al. (1996) and its relatively small size at first capture (Table 1) this shark was likely still immature when recaptured. J007072 had no length measurement recorded upon recapture, however based on its low mass (Table 1) this individual was also very likely immature upon recapture, meaning that seven of the nine recaptured individuals were most likely still immature, with one mature individual and one of unknown maturity status (Table 1). Figure 4. Open in new tabDownload slide Capture and recapture locations for all recaptured porbeagles (except J000061) tagged in Irish waters (n = 8). (a) Sharks recaptured in Irish coastal waters, and (b) sharks recaptured outside Irish coastal waters. Note: no track is given for shark J040015 due to the close proximity of its capture and recapture locations. Figure 4. Open in new tabDownload slide Capture and recapture locations for all recaptured porbeagles (except J000061) tagged in Irish waters (n = 8). (a) Sharks recaptured in Irish coastal waters, and (b) sharks recaptured outside Irish coastal waters. Note: no track is given for shark J040015 due to the close proximity of its capture and recapture locations. Table 1. Capture locations, dates and biometric information for all recaptured porbeagles, including initial capture data. Tag ID J000061 J000814 J003682 J007068 J007072 J023900 J026819 J043160 J040015 Sex F U U U U U U F F Capture  Location Kinsale Inishmaan Courtmacsherry Liscannor Liscannor Cork Harbour Dunfanaghy Red Bay Dunaff  Lat, Long 51.617°, −8.417° 53.069°, −9.620° 51.370°, −8.500° 52.917°, −9.416° 52.917°, −9.416° 51.760°, −8.241° 55.359°, −7.957° 55.075°, −6.037° 55.291°, −7.55°  Date 5 Aug 1972 3 Aug 1970 24 Jul 1978 27 Aug 1986 31 Aug 1986 28 Jun 1997 26 Sep 2008 12 Sep 2012 16 Aug 2009  TL (cm) NA NA NA 90 NA NA 110 160 149.6a  FL (cm) NA NA NA 82.53a NA NA 95 145.33a 136  Mass (Kg) 9.071 NA 15.8 NA 22.6 15.8 NA NA NA Recapture  Days At Liberty 3742 71 3946 301 780 525 273 311 3156  Location Grand Banks Celtic Sea Suduroy Faroe Killala Bay Achill Island Faroe Islands Stornoway Malin Head Malin Head  Lat, Long 44.933°, −52.417° 47.378°, −9.590° 61.354°, −6.777° 54.330°, −9.310° 53.910°, −10.280° 60.601°, −9.182° 58.233°, −6.267° 55.317°, −7.467° 55.4° −7.45°b  Date 3 Nov 1982 13 Oct 1970 13 May 1989 24 Jun 1987 19 Oct 1988 5 Dec 2009 26 Jun 2009 20 Jul 2013 7 Apr 2018  TL (cm) 185 170 NA 133.9a NA 150 NA 182.00 250.00  FL (cm) 167.76a 154.30a NA 121.92 NA 136.36a NA 165.07a 226.07a  Mass (Kg) 100 38 90 38.1 34 approx. 20 NA 59 NA  Estimated maturity Immature Immature Unknown Immature Immature Immature Immature Immature Mature  Recapture method Longline Unknown Trawler Drift net Tangle net Trammel net Unknown Unknown Unknown Tag ID J000061 J000814 J003682 J007068 J007072 J023900 J026819 J043160 J040015 Sex F U U U U U U F F Capture  Location Kinsale Inishmaan Courtmacsherry Liscannor Liscannor Cork Harbour Dunfanaghy Red Bay Dunaff  Lat, Long 51.617°, −8.417° 53.069°, −9.620° 51.370°, −8.500° 52.917°, −9.416° 52.917°, −9.416° 51.760°, −8.241° 55.359°, −7.957° 55.075°, −6.037° 55.291°, −7.55°  Date 5 Aug 1972 3 Aug 1970 24 Jul 1978 27 Aug 1986 31 Aug 1986 28 Jun 1997 26 Sep 2008 12 Sep 2012 16 Aug 2009  TL (cm) NA NA NA 90 NA NA 110 160 149.6a  FL (cm) NA NA NA 82.53a NA NA 95 145.33a 136  Mass (Kg) 9.071 NA 15.8 NA 22.6 15.8 NA NA NA Recapture  Days At Liberty 3742 71 3946 301 780 525 273 311 3156  Location Grand Banks Celtic Sea Suduroy Faroe Killala Bay Achill Island Faroe Islands Stornoway Malin Head Malin Head  Lat, Long 44.933°, −52.417° 47.378°, −9.590° 61.354°, −6.777° 54.330°, −9.310° 53.910°, −10.280° 60.601°, −9.182° 58.233°, −6.267° 55.317°, −7.467° 55.4° −7.45°b  Date 3 Nov 1982 13 Oct 1970 13 May 1989 24 Jun 1987 19 Oct 1988 5 Dec 2009 26 Jun 2009 20 Jul 2013 7 Apr 2018  TL (cm) 185 170 NA 133.9a NA 150 NA 182.00 250.00  FL (cm) 167.76a 154.30a NA 121.92 NA 136.36a NA 165.07a 226.07a  Mass (Kg) 100 38 90 38.1 34 approx. 20 NA 59 NA  Estimated maturity Immature Immature Unknown Immature Immature Immature Immature Immature Mature  Recapture method Longline Unknown Trawler Drift net Tangle net Trammel net Unknown Unknown Unknown asignifies where total length (TL) has been calculated from fork length (FL) or vice versa using the equation from Kohler et al. (1996). bthe recapture coordinates for shark J040015 are estimated as only a general descriptive location was given. Open in new tab Table 1. Capture locations, dates and biometric information for all recaptured porbeagles, including initial capture data. Tag ID J000061 J000814 J003682 J007068 J007072 J023900 J026819 J043160 J040015 Sex F U U U U U U F F Capture  Location Kinsale Inishmaan Courtmacsherry Liscannor Liscannor Cork Harbour Dunfanaghy Red Bay Dunaff  Lat, Long 51.617°, −8.417° 53.069°, −9.620° 51.370°, −8.500° 52.917°, −9.416° 52.917°, −9.416° 51.760°, −8.241° 55.359°, −7.957° 55.075°, −6.037° 55.291°, −7.55°  Date 5 Aug 1972 3 Aug 1970 24 Jul 1978 27 Aug 1986 31 Aug 1986 28 Jun 1997 26 Sep 2008 12 Sep 2012 16 Aug 2009  TL (cm) NA NA NA 90 NA NA 110 160 149.6a  FL (cm) NA NA NA 82.53a NA NA 95 145.33a 136  Mass (Kg) 9.071 NA 15.8 NA 22.6 15.8 NA NA NA Recapture  Days At Liberty 3742 71 3946 301 780 525 273 311 3156  Location Grand Banks Celtic Sea Suduroy Faroe Killala Bay Achill Island Faroe Islands Stornoway Malin Head Malin Head  Lat, Long 44.933°, −52.417° 47.378°, −9.590° 61.354°, −6.777° 54.330°, −9.310° 53.910°, −10.280° 60.601°, −9.182° 58.233°, −6.267° 55.317°, −7.467° 55.4° −7.45°b  Date 3 Nov 1982 13 Oct 1970 13 May 1989 24 Jun 1987 19 Oct 1988 5 Dec 2009 26 Jun 2009 20 Jul 2013 7 Apr 2018  TL (cm) 185 170 NA 133.9a NA 150 NA 182.00 250.00  FL (cm) 167.76a 154.30a NA 121.92 NA 136.36a NA 165.07a 226.07a  Mass (Kg) 100 38 90 38.1 34 approx. 20 NA 59 NA  Estimated maturity Immature Immature Unknown Immature Immature Immature Immature Immature Mature  Recapture method Longline Unknown Trawler Drift net Tangle net Trammel net Unknown Unknown Unknown Tag ID J000061 J000814 J003682 J007068 J007072 J023900 J026819 J043160 J040015 Sex F U U U U U U F F Capture  Location Kinsale Inishmaan Courtmacsherry Liscannor Liscannor Cork Harbour Dunfanaghy Red Bay Dunaff  Lat, Long 51.617°, −8.417° 53.069°, −9.620° 51.370°, −8.500° 52.917°, −9.416° 52.917°, −9.416° 51.760°, −8.241° 55.359°, −7.957° 55.075°, −6.037° 55.291°, −7.55°  Date 5 Aug 1972 3 Aug 1970 24 Jul 1978 27 Aug 1986 31 Aug 1986 28 Jun 1997 26 Sep 2008 12 Sep 2012 16 Aug 2009  TL (cm) NA NA NA 90 NA NA 110 160 149.6a  FL (cm) NA NA NA 82.53a NA NA 95 145.33a 136  Mass (Kg) 9.071 NA 15.8 NA 22.6 15.8 NA NA NA Recapture  Days At Liberty 3742 71 3946 301 780 525 273 311 3156  Location Grand Banks Celtic Sea Suduroy Faroe Killala Bay Achill Island Faroe Islands Stornoway Malin Head Malin Head  Lat, Long 44.933°, −52.417° 47.378°, −9.590° 61.354°, −6.777° 54.330°, −9.310° 53.910°, −10.280° 60.601°, −9.182° 58.233°, −6.267° 55.317°, −7.467° 55.4° −7.45°b  Date 3 Nov 1982 13 Oct 1970 13 May 1989 24 Jun 1987 19 Oct 1988 5 Dec 2009 26 Jun 2009 20 Jul 2013 7 Apr 2018  TL (cm) 185 170 NA 133.9a NA 150 NA 182.00 250.00  FL (cm) 167.76a 154.30a NA 121.92 NA 136.36a NA 165.07a 226.07a  Mass (Kg) 100 38 90 38.1 34 approx. 20 NA 59 NA  Estimated maturity Immature Immature Unknown Immature Immature Immature Immature Immature Mature  Recapture method Longline Unknown Trawler Drift net Tangle net Trammel net Unknown Unknown Unknown asignifies where total length (TL) has been calculated from fork length (FL) or vice versa using the equation from Kohler et al. (1996). bthe recapture coordinates for shark J040015 are estimated as only a general descriptive location was given. Open in new tab The ninth recapture (J000061) was taken in a longline fishery on the Grand Banks of Newfoundland in November 1982 over 10 years after first capture in August 1972 near Kinsale and is discussed in more detail in Cameron et al. (2018). A tenth tag was washed up on a beach on the Ile de Ré on the west coast of France in April 1979, 645 days after being attached to a shark in June 1977 at Kinsale. It is unknown how this tag became detached. Discussion Our results show the great majority of the porbeagle catch in Irish waters likely consists of juveniles, with very few adults present. These results also provide clues as to the wider population structure and movement patterns of the northeast Atlantic subpopulation, providing evidence for size/age-based spatial segregation. Recapture data provide a limited degree of evidence for potential long-term site fidelity and in some cases contradict previously suggested general migration patterns. This and other studies demonstrate the value in data gathered by utilizing the recreational angling sector and our research will add to the growing list of such work (e.g. Kohler et al., 1998; Gartside et al., 1999; Boucek and Rehage, 2015; Fetterplace et al., 2018; Näslund and Lundgren, 2018). The FL-based maturity estimates support the hypothesis (H1) that the population structure of porbeagles in Irish waters would be biased toward juveniles. Here, we found only eight individuals that exceeded the minimum length at maturity, which could suggest that larger mature sharks in the northeast Atlantic population may move to higher latitudes than juveniles, as has been observed in both the Pacific and northwestern Atlantic (e.g. Aasen, 1963; Francis, 2016). For example, in the south Pacific, large females, which attain greater sizes than males, are only infrequently caught as bycatch from fisheries around the New Zealand coast in comparison to juveniles (Semba et al., 2013; Francis, 2016). Likewise, in the northwest Atlantic larger individuals appear to migrate to higher latitudes during the summer months (Aasen, 1963). In our study, the recapture of at least one juvenile porbeagle as far north as the Faroe Islands (60.601°, −9.182°) indicates that juveniles will move to higher latitudes; however, the second of the two fish recaptured from the Faroes (J023900) was likely mature or at least nearing maturity based on FL. This apparent segregation by latitude may be explained by the presence of the retia mirabilia, a structure of closely associated arteries and veins that allows porbeagles to maintain body temperatures elevated above the surrounding water temperature (Carey and Teal, 1969; Anderson et al., 2001; Goldman et al., 2004). The thermoregulatory ability of lamnid sharks has only recently been subject to more extensive study (see Watanabe et al., 2015), but smaller porbeagles may be restricted to lower latitudes owing to their possibly lesser thermoregulatory abilities. The possible presence of latitudinal segregation in the northeast Atlantic does not, by itself, entirely explain why so few mature individuals were tagged since adults would still be expected to pass through more southerly waters, such as those around Ireland, during their annual migrations (Biais et al., 2017). Previous pop-up satellite archival tag (PSAT) tagging of adult porbeagles around the Bay of Biscay and southwest Ireland shelf break indicated that northward movement could occur at any period from August to October with further northern or westward movement from September-January and a southern return from January to April (Biais et al., 2017). In the above study at least five of the nine tagged individuals entered Irish waters, with one individual passing through Irish coastal waters near the western group in July. However, the majority remained much further offshore, outside angling hotspots, than those tagged in this study, and/or passed through Irish waters outside of the period from June to October when the majority of captures were recorded. There is also anecdotal evidence of a larger specimen being caught in a bottom-trawl in early July in the southern Irish Sea (Spain, 2009), suggesting that the selectivity of angling tactics by recreational fisherman (e.g. gear, depth, bait) could potentially reduce the catchability of larger sharks. Historic data from Scottish and Spanish waters show a higher incidence of larger individuals caught using commercial fishing gear, predominantly in bottom trawls, long lines, seine nets, and gill nets in Scottish waters and in longlines in the Spanish swordfish fishery (Mejuto, 1985; Gauld, 1989). Thus, there is a strong implication that fishing gear may play a role in observed population structure. The coastal distribution and timing of porbeagle captures in this study provides some support for the previously reported migration patterns of porbeagles, with sharks generally residing in shallower coastal waters during the summer (Campana et al., 2008; Pade et al., 2009; Saunders et al., 2011; Biais et al., 2017). The timings of captures seem to fit well with the pattern of northward movement from August to October described by Biais et al. (2017), although they must be taken lightly given the degree of overlap in the credible intervals for the capture dates of each region. Furthermore, it must be noted that, in the absence of catch per unit effort (CPUE) data, it is difficult to disentangle the porbeagles’ distribution and movements in summer months from the distribution and timing of recreational shark angling in Ireland, as this tends to be a summer activity (W. Roche, pers. comm.). Moreover, Irish charter boats and private recreational boats carry out day trips when angling for porbeagles, and hence are generally restricted to coastal seas within a 25-km radius of their Irish port of origin (W. Roche, pers. comm.). Therefore, the coastal location of most captures may reflect this restriction of angling activity. The spatio-temporal pattern of recaptures, with one clear exception, appears to agree with the general annual migration pattern described by Biais et al. (2017). The recaptures of sharks J007072 and J023900 match the pattern of northward movement in summer and autumn, southerly movement from January to April and return to the Bay of Biscay and southwest Ireland shelf break between March and June, as described by Biais et al. (2017). The recaptures of sharks J007068 and J026819 in June, from Killala Bay, Ireland, and Stornoway, Scotland respectively, suggest northward movement may occur earlier than previously described. The recapture of shark J000814 from the Celtic Sea in October suggests that southern movement described by Biais et al. (2017) may also occur at an earlier date in some individuals, with the caveat that we sampled a different size cohort and maturity class. The recapture of shark J003682 from the Faroe Islands in May 1989 stands in contrast to this migration pattern, occurring almost a month before the earliest captures from the north coast of Ireland, when porbeagles would be expected to be ∼1400 km further south. Although other studies have noted highly variable migratory patterns in porbeagles, this result is an extreme deviation from the generally observed migration patterns for this species (Campana et al., 1999, 2002; Campana and Joyce, 2004; Francis et al., 2008; Pade et al., 2009; Saunders et al., 2011; Biais et al., 2017). Previous studies have indicated that porbeagles show site fidelity, both short-term over the course of a single season during the period of summer residency in shallow coastal waters, and long term when returning to the same overwintering grounds in subsequent years (Pade et al., 2009; Saunders et al., 2011; Biais et al., 2017). Our results suggest that porbeagles may also show long-term site fidelity to summer residency areas in subsequent years, with sharks J007068, J007072, J043160, and J040015 all recaptured from locations off the coast of Ireland within 200 km of their original capture locations between 10 months and 8 years later. If site fidelity is proven, this could enable more precise conservation measures such as commercial fishing restrictions in key aggregation sites to reduce bycatch, as suggested for other elasmobranch stocks (Barker and Schluessel, 2005). This may be of lesser relevance within Europe where a zero total allowable catch was implemented in 2010 [Council Regulations (EU) 2015/104, 2016/72, 2017/127, and 2018/120, in ICES 2016], and combined landings of both porbeagles and unspecified “Mackerel sharks” (family Lamnidae) have not exceeded 10 metric tonnes since 2014 (European Commission, 2018). However, such measures could have utility outside of EU waters where limited conservation measures are in place for this species. In the northern group, the apparent shift towards earlier capture dates was unrelated to annual variation in SST. Although our analysis only included a rudimentary measure of SST and lacked CPUE data, previous studies have found SST to be a poor predictor of porbeagle distribution (immature and mature individuals) in the northwest Atlantic population (Campana and Joyce, 2004). The possible lack of a strong link between capture date and SST indicates there may be another driving factor for this shift. This may simply be a result of a change in the timing of angling effort or it may indeed reflect some shift in porbeagle movement patterns caused by environmental factors, such as changes in the distribution of prey species. In the northwest Atlantic population, juveniles are thought to feed primarily on cephalopods and small pelagic fish (e.g. Atlantic mackerel, Scomber scombrus and Herring, Clupea harengus), while adults may feed more upon groundfish (Gauld, 1989; Joyce et al., 2002). The potential northward movement of the porbeagles corresponds well with mackerel movement from spawning grounds south of Ireland to their northern feeding grounds in the summer (Uriarte et al., 2001), suggesting that this may be a driving factor in the possible northward movement proposed here. However, owing to the lack of CPUE in these recreational porbeagle fisheries, the possible northward movement of juveniles and the apparent shift towards earlier capture dates in the northern group must be interpreted with caution. Conclusions Our findings underline the value of mark-recapture programmes for the study of this and other wide-ranging marine species and should act as a driver for greater participation in, more widespread implementation of, and better integration and knowledge sharing between such programmes, as established in the northwest Atlantic (Campana et al., 1999, 2002; Kohler et al., 2002). Tagging of more porbeagles (with conventional and/or data logging tags) across a greater proportion of their range in targeted sites across the northeast Atlantic would allow scientists to draw more firm conclusions about the movement patterns of this species. Even with a limited number of tagged individuals, our findings appear to indicate that Irish waters may act as an important summer aggregation site for juvenile porbeagles, with sharks potentially moving northward throughout the duration of the summer. Recapture locations indicate porbeagle individuals may return to nearby locations between years. These consistent spatial and temporal results also provide evidence to suggest that, in at least some cases, juvenile movements may be more extensive and unpredictable than previously considered, potentially placing them at higher risk of fishery-related mortality outside EU waters. For example, two individuals from this study were recaptured from commercial fishing vessels in Faroese waters. This highlights the need for an international management strategy, which protects porbeagles outside of regulated European waters [Council Regulation (EU) 2015/104, 2016/72, 2017/127, and 2018/120, in ICES 2016], particularly since size-based spatial segregation may further increase the risk of overfishing in this species. This impact has already been shown in the southern hemisphere where disproportionate juvenile mortality in fisheries has been noted (Francis, 2016). Spatial segregation in the form suggested here could also plausibly lead to disproportionate adult mortality outside protected EU waters. Supplementary data Supplementary material is available at the ICESJMS online version of the manuscript. 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TI - Population structure and spatial distribution of porbeagles (Lamna nasus) in Irish waters
JF - ICES Journal of Marine Science
DO - 10.1093/icesjms/fsz046
DA - 2019-12-01
UR - https://www.deepdyve.com/lp/oxford-university-press/population-structure-and-spatial-distribution-of-porbeagles-lamna-pAHfMRrkg2
SP - 1581
VL - 76
IS - 6
DP - DeepDyve
ER -