Long-term changes in age at sexual maturity of Northeast Arctic cod (Gadus morhua L.)Jørgensen, Terje
doi: 10.1093/icesjms/46.3.235pmid: N/A
Maturity ogives for the Northeast Arctic cod for year classes 1923 to 1976 were computed from the age and number of spawning zones in otoliths of cod caught on the spawning grounds at Lofoten. A reduction in the median age at maturity of approximately 2.5 years over the period was found. The change has primarily affected year classes 1950 and later. A reduction in the mean age of the spawning stock by approximately 2.5 years was also found. Males generally matured at a lower age than females, but the difference has become smaller over the period studied. It is suggested that the reduction in median age at maturity is primarily a compensatory response to the reduced stock size. Higher fishing pressure on the late maturing individuals than on the early maturing individuals may also have contributed to the changes, with or without the mediation of genetic changes.
A length-structured population model for southern fin whales and a test for density dependenceSampson, David B.
doi: 10.1093/icesjms/46.3.249pmid: N/A
It is reasonable to assume that the populations of baleen whales in the Southern Hemisphere are limited by prey availability more than by disease or predation and that the increased food per capita, a likely result of the rapid removal of animals by whaling operations during this century, may have affected the whale populations'rates of birth, juvenile mortality, and growth. This paper assesses the validity of these assumptions by attempting to detect changes in these rates and to ascertain which of them could have been responsible for density-dependent regulation in two populations of southern fin whales (Balaenoptera physalus).It is assumed that the birth and death rates for animals in these populations depend on the size of the individuals; larger females are on average more frequently pregnant; infants and juveniles which are unusually small or slow growing may be subject to higher levels of natural mortality; animals less than the minimum size are not harvested by whaling operations. The rate of progression from birth to adulthood and reproduction is governed by the rate of growth-in-length. An increase in the rate of recruitment will cause a decrease in the average length.A length-structured population model is developed from Deriso's delay-difference equation model; two difference equations describe the annual changes in the numbers of females greater than the legal minimum length and the annual changes in their combined lengths. Reproduction is modelled as a function of average length. Parameter estimates are obtained from observed catches, times spent whaling, average lengths of the catches, and the fractions pregnant.For the period examined. 1931–1975. more animals were caught than were produced; the populations declined steadily. It is assumed that if there were temporal trends in the population parameters during this period in response to declining density, they were approximately linear. Density dependence is measured by changes in goodness-of-fit for the model with and without trends in the parameters which govern growth-in-length, pregnancy rate, and juvenile survival. Goodness-of-fit is significantly improved by introducing changes in the pregnancy rate, but not with changes in juvenile survival or growth-in-length. It is therefore concluded that the populations of southern fin whales are regulated mainly or exclusively by densitydependent changes in the rate of reproduction
Comparative studies of coastal pelagic fish reproductive habitats: the Brazilian sardine (Sardinella aurita)Bakun, Andrew; Parrish, Richard H.
doi: 10.1093/icesjms/46.3.269pmid: N/A
We extend a framework of comparative climatology of reproductive habitats of neritic pelagic fishes, heretofore focused on temperate eastern ocean upwelling systems, by adding a tropical western ocean example. Maritime weather reports off southeastern Brazil are summarized to yield distributions of wind stress, Ekman transport, wind mixing index, insolation, cloud cover and sea surface temperature for two-month segments of the seasonal cycle. These are used to describe seasonal aspects of the oceanography of the region. Vigorous coastal upwelling occurs in the vicinity of Cabo Frio-Cabo Sāo Tomé through most of the year, relaxing only during austral fall. Directly downstream of this upwelling center lies the coastal bight between Cabo Frio and Cabo Santa Marta Grande, within which offshore Ekman transport and wind-induced turbulent mixing fall to coastwise minima. The near-coastal water column within the bight, while vertically homogeneous during winter, becomes stably stratified during summer. We infer a rather enclosed circulation pattern within the bight, with the main Brazil Current flow skirting across the bight opening rather than following the coastline into the bight interior. This coastal bight constitutes the primary reproductive habitat of the Brazilian sardine. Peak spawning activity during summer serves to place larvae into a stable, enriched environment, where they enjoy high likelihood of retention. In the manner of achieving these advantages, the reproductive strategy of this population appears to be a nearly exact analogue to those of sardine populations inhabiting eastern boundary current upwelling systems, in spite of its western ocean boundary location and the fact that a warmer water genus is involved. The conclusion is that this Sardinella population is solving similar dominant environmental problems to those faced by the more temperate Sardinops and Sardina stocks of eastern boundary systems, only at a warmer temperature range. These findings lend support to the idea that our various fragments of experience of environmental effects on fish populations are not unconnected anecdotes, as they are often treated, but are amenable to scientific generalization.
Growth-maturation interactions of Acadian redfish (Sebastes fasciatus Storer) in the Gulf of Maine-Georges Bank region of the Northwest AtlanticMayo, R. K.; Burnett, J.; Smith, T. D.; Muchant, C. A.
doi: 10.1093/icesjms/46.3.287pmid: N/A
Growth and maturation rates were derived for redfish (Sebastes fasciatus Storer) from data collected on bottom trawl surveys conducted in the Gulf of Maine-Georges Bank region between 1975 and 1980. Estimates of Lx ranged from 32 to 35 cm for males and 37 to 41 cm for females, depending on season and location. Growth coefficients (K) were higher for males (0.158 to 0.175) than for females (0.144 to 0.148), but were consistently lower for both sexes in shallower inshore regions of the western Gulf of Maine. Instantaneous growth rates were also lower for younger fish of both sexes inshore. Differences in linearized growth rates between males and females were highly significant (p<0.01), while seasonal effects were generally not significant (p>0.05). Distinct differences in spatial and temporal distribution patterns of redfish larvae were noted between the eastern and central regions of the Gulf of Maine; it is suggested that larval transport may influence juvenile growth. The maturation process commenced as early as age 3 for some individuals and was essentially complete by age 10 for both sexes. The overall median size at maturity was slightly lower for males (20.9 cm) than for females (22.3), although the median age at maturity (5.5 years) was similar for both sexes. However, estimates of both median age and size at maturity were considerably lower in the shallower inshore area than in the offshore region, suggesting a linkage between accelerated maturation and reduced growth rates of immature rednsh in the inshore region. Interactions between early growth and maturation were investigated using a two-factor logistic model incorporating size and age. Both factors accounted for a significant amount of the variation in the proportion mature for males and females in the offshore Gulf of Maine and for females inshore. For inshore males, however, maturation appeared to be almost exclusively age-driven. The two-factor model, which provides a visual representation of the maturation surface, can provide an objective basis for evaluating the role of size and age in the maturation process. It may also be a useful tool for systematic comparisons when applied to several species exhibiting various life history strategies.
Demography and distribution of the Patagonian squid (Loligo gahi d'Orbigny) during the austral winterHatfield, E. M. C.; Rodhouse, P. G.; Porebski, J.
doi: 10.1093/icesjms/46.3.306pmid: N/A
A bottom-trawl survey of the Falklands Interim Conservation Zone was carried out during July and August 1988. Samples of Loligo gahi were collected from depths of 50–400 m and analysed for size composition, maturity, and stomach fullness. L. gahi increased in size and maturity with depths >100 m. Immature and fully mature squid were found at depths ≤100 m. The data support the hypothesis that this species spawns in shallow water and migrates down the continental shelf and continental slope after hatching. It then returns to shallow water to spawn. Highest concentrations of squid appeared to occur at depths and in areas where the commercial fishery operates. Samples were only taken during daylight hours, and stomach fullness was at its highest in the mid-afternoon. Several of the life history features resemble closely those of other loliginid species.
Identification of fish echoes located at 65 km range by shore-based sonarRevie, J.; Weston, D. E.; Jones, F. R. Harden; Fox, G. P.
doi: 10.1093/icesjms/46.3.313pmid: N/A
In June 1967. the Fisheries Laboratory, Lowestoft. cooperated with the Admiralty Research Laboratory to identify fish detected by the long-range correlation sonar equipment installed at the Admiralty Experimental Station. Perranporth. The RV “Corella” was directed from the shore station to an area some 65 km distant, which was returning a strong and extended echo. The “Corella” was able to correlate this target with shoals detected on a conventional echo sounder, and substantial catches of sprats (Sprattus sprattus L.) were taken in the area using a midwater trawl. Comparison is made of the behaviour of the shoals at dusk and dawn as observed by echo sounder and by long-range sonar. The movement of the target over a six-day period agreed well with that expected from tidal movement.
A stochastic integrated VPA for herring in the Baltic Sea using acoustic estimates as auxiliary information for estimating natural mortalitySparholt, Henrik
doi: 10.1093/icesjms/46.3.325pmid: N/A
Acoustic estimates on stock numbers at age for herring in the Central Baltic Sea are used in a stochastic integrated VPA to obtain estimates of mean natural mortality rates by ages for 1981–1987. The estimates of the natural mortality rates are inconsistent with the results of the ICES Multispecies VPA (MSVPA) carried out for the Central Baltic Sea. The high natural mortality rates of the young ages estimated by the MSVPA are not confirmed by the present analysis, where the natural mortality rates are estimated to be the same (0.18 year−1) for ages 1 to 9. It is concluded that the acoustic survey underestimates the abundance of ages 1, 2, and 3. However, the actual natural mortality might lie somewhere between the estimates from the MSVPA and those presented here, because the MSVPA could be wrong by as much as a factor of 2 owing to uncertainties in the consumption data used for cod in the MSVPA.