ICES Journal of Marine Science: Journal du Conseil

Shepherd, J. G.

doi: 10.1006/jmsc.1997.0222pmid: N/A

The analysis of multiple time series of indices of recruitment to fish stocks by means of calibration regression is discussed, together with the use of the relationships so fitted for the prediction of year class strength. A simple method for the combination of the estimates derived from different index series using inverse variance weighted averages is proposed, and methods for the estimation of the overall error in the prediction are discussed. The method used has been shown to perform well in simulation tests, and is well adapted for use on real datasets with time series of variable length and missing data. It has been implemented in a computer program (RCT3, superseding RCRTINX2) which is available for operational use, and has been endorsed by the ICES Working Group on the Methods of Fish Stock Assessment as being satisfactory for operational use until more complex methods have been shown to have superior performance.opyright 1997 International Council for the Exploration of the Sea

Danielssen, D. S.; Edler, L.; Fonselius, S.; Hernroth, L.; Ostrowski, M.; Svendsen, E.; Talpsepp, L.

doi: 10.1006/jmsc.1996.0210pmid: N/A

The Skagerrak Experiment (SKAGEX), was a large, international, ICES-supported joint venture, carried out in the Skagerrak–Kattegat area on four different surveys in the period 1990–1991. It involved some 20 institutes and, at times, up to 17 research vessels. The main aim of the Experiment was to identify and quantify the different water masses entering and leaving the Skagerrak area and their variation over time. It also aimed to investigate the mechanisms that drive the circulation and to study their effects on biological processes. The aim was to be attained mostly through extensive synoptic observations. This paper focuses on the variability in physical, chemical and biological parameters during the first part of SKAGEX, 24 May–20 June 1990. During the first half of the period of investigation, the main outflow from the Skagerrak, represented by the Norwegian Coastal Current, was barotropic with daily mean velocities varying from 10–40 cm s−1. During the second half a clear baroclinic current component developed, giving rise to near surface velocities of up to 100 cm s−1. A pronounced feature in the Skagerrak during the study was the counter-clockwise circulation of the Norwegian Coastal Current at times of strong northwesterly winds. During such conditions this surface water reached as far as the Danish coast south of 57°N and upwelling along the Norwegian coast was also found. During northerly winds upwelling also occurred along the Swedish coast. The nutrient-rich Jutland Coastal Water, originating from the German Bight, was never found to reach the inner part of the Skagerrak during this first part of SKAGEX. It was partly blocked or diluted by other water-masses. A large “ridge” of nutrient-rich Atlantic water was found in the central Skagerrak throughout the investigation. It is shown that this elongated “ridge” was associated with the deepest (>500 m) area of the Skagerrak. Within this area, high subsurface chlorophyll concentrations were always found and, due to the persistence of the supply of nutrients, it is concluded that this phenomenon could be one of the main reasons for the high productivity of the Skagerrak.

Stergiou, K. I.; Politou, C.-Y.; Christou, E. D.; Petrakis, G.

doi: 10.1006/jmsc.1997.0231pmid: N/A

In the present study we compare the overall performance of the 14 mm (from knot-to-knot) diamond-shaped codend (14D), presently used in Hellenic waters, with those of 20 mm square-shaped (20S) and 20 mm diamond-shaped (20D) trawl codends. The experiments took place at 12 stations in the western Aegean Sea and were based on the covered codend method (cover codend: 10 mm from knot-to-knot, 10D). The results of applying multivariate techniques, to the different station/codend combinations suggest that they form groups which differ from each other in terms of species composition, species diversity, and catch numbers/weights per hour of fishing. In general, the 10D covers of 20D and 20S were separated from the 10D covers of 14D. The 20D and 20S codends were generally grouped together and separated from those of 14D and 10D. Finally, the 14D codend either form groups by themselves or group together with the 10D cover codends. In addition, the results indicated that the 20D and 20S codends, when compared with 14D, allow significantly more individuals and number of species to escape through the meshes and are characterised by considerably higher commerical/non-commercial ratios in terms of both weight and number. In absolute numbers and weights 14D caught a very large quantity of non-commercial species whereas the weight of the commercial species caught was among the lowest. The commercial/non-commercial ratios of 20D were higher than those of 20S and, in absolute numbers and weights, 20S caught relatively more non-commercial species than 20D. These facts clearly indicate that 14D is harmful in terms of species diversity for the Hellenic demersal stocks and hence its use should be prohibited.

Thórarinsson, Kristján; Jóhannesson, Gardar

doi: 10.1006/jmsc.1996.0199pmid: N/A

A simple model relating catchability to maturity is presented, which allows for computation of annual age-specific correction factors for groundfish survey indices. The model is based on the hypothesis that mature fish are less catchable than immature fish during the spawning season and are therefore less available to groundfish surveys carried out during that period. The effect modelled is an interaction effect – its operation requires both a change in catchability at maturity and temporal changes in maturity at age in the stock. In recent years, assessments of the Icelandic cod stock have relied on VPAs calibrated (tuned) with bottom trawl c.p.u.e. data. The c.p.u.e. data are from the commercial trawler fleet and from the Marine Research Institute's annual groundfish survey conducted in March, covering the entire distribution area of the stock around Iceland. At the time of the survey, most of the year's spawners have migrated towards the spawning grounds. Independent data from commercial catches indicate that the proportions of fish mature at age in the Icelandic cod stock have increased substantially in recent years. Furthermore, comparison of proportions mature in the catches with survey indices for mature and immature fish indicate that immature fish are substantially more catchable than mature fish of the same age. The effects of changes in proportion mature on estimated stock numbers are discussed with reference to limitations in the available data.

Walker, Paddy; Howlett, Gary; Millner, Richard

doi: 10.1006/jmsc.1997.0223pmid: N/A

Tagging experiments on three ray species, thornback ray (Raja clavata), spotted ray (R. montagui) and starry ray (R. radiata), carried out between 1959 and 1976 have been analysed, and distribution and movements are described. Within the North Sea each species belonged to a single stock. For the thornback and spotted rays a separate stock could be identified in the eastern English Channel. After an initial period of dispersion the three species remained resident within an average of 30–50 nautical miles (North Sea) or 10–15 nautical miles (English Channel) of the release area. Adult thornback rays migrated from offshore, relatively deep (30 m) water in the winter months to inshore, shallow (10 m) water in the summer months, probably to mate and spawn.

Kalikhman, I.; Ostrovsky, I.

doi: 10.1006/jmsc.1996.0205pmid: N/A

A mathematical model (Kriging) was used to examine the effects of acoustic survey design on the adequacy of reconstruction of patchy distribution fields. The model simulates fish (or plankton) patches of different shapes and spatial orientations, and an acoustic survey by parallel transects. For a series of mathematical experiments, the patchy fields were reconstructed using simulated survey data. Conformity of the reconstructed fields to those originally generated was evaluated by calculating their correlations. The experiments indicate that a survey in the direction of patch elongation gives best results. The optimal ratio of the distance between transects to the autocorrelation radius for the field in the direction of surveying has been determined. Lack of a priori information on patch orientation may considerably reduce the quality of survey design. A posteriori determination of patch orientation allows reconstruction of the best field, attainable on the basis of the data from the survey conducted. In addition, surveys of moving patches are considered.

Orr, D. C.; Bowering, W. R.

doi: 10.1006/jmsc.1996.0198pmid: N/A

Pelagic invertebrates [northern shrimp (Pandalus borealis), Cephalopoda, Mysidacea, Amphipoda and Euphausiacea] and fish [redfish (Sebastes mentella), R. hippoglossoides and Arctic cod (Boreogadus saida)] were the most important food items found in 4295 Greenland halibut (R. hippoglossoides) stomachs collected from Davis Strait [Northwest Atlantic Fisheries Organization (NAFO) Subareas 0 and 1], during 1986. Ordination methods indicated that predator size and capture depth accounted for 85.2% of the variation in diet, while classification methods were used to illustrate these relationships. Additionally, the study extended into locations not previously sampled, therefore, knowledge was expanded upon pertaining to the distribution ranges of important prey.

Magnússon, J. V.; Jóhannesson, G.

doi: 10.1006/jmsc.1996.0197pmid: N/A

Annual variations in abundance and distribution of 0-group redfish in the Irminger Sea have been recorded since 1970. These are reviewed for the time period 1972 to 1994 and relations are investigated between abundance indices based on the 0-group survey and a groundfish survey. The abundance of 0-group redfish in the Irminger Sea and around Iceland varies considerably from year to year. Around Iceland, the abundance is generally low and depends on the influx of warm Atlantic water into the area north of Iceland. The upper layer temperatures in the Irminger Sea are normally 7°–9°C. The 0-group redfish off North Iceland belongs to Sebastes marinus. In the Irminger Sea, the proportion of this species is highest in the sub-areas East Greenland North and Central Irminger Sea North. Length-based abundance indices from the Icelandic groundfish survey (which cover the Icelandic shelf) are available for S. marinus from 1985 to 1994. From these indices, a recruitment index was constructed, representing the abundance of 31–35 cmS. marinus. Possible relationships between the recruitment index and the 0-group were investigated by considering different time-lags (10–15 years) between the 0-group appearance and its recruitment to the fishery. A significant relationship with R2 from 0.54 to 0.71 was found by assuming a time-lag of 13 years between the 0-group and the recruitment index.

Bromley, P. J.; Watson, T.; Hislop, J. R. G.

doi: 10.1006/jmsc.1996.0211pmid: N/A

Stomach content analysis was used to investigate feeding interactions between pelagic 0-group gadoids in the northern North Sea. The species studied were cod (Gadus morhua L.), haddock (Melanogrammus aeglefinus L.), whiting (Merlangius merlangus L.), saithe (Pollachius virens L.) and Norway pout (Trisopterus esmarkii Nilsson). The fish were caught at a site south-east of Shetland in late spring during 24 h depth-stratified fishing using a mid-water trawl. There was overlap in the range of prey consumed by the various species. Fish of 1–3 cm in length fed mainly on copepods and other invertebrates. Larger Norway pout continued to feed almost entirely on copepods, but the other species diversified as they grew to include fish in their diet, cod and whiting becoming almost entirely piscivorous. Haddock and saithe ate moderate amounts of fish combined with crustacea in roughly equal proportions. The depth at which the fish were caught did not appear to influence the range of prey consumed, but fish was slightly more prevalent in the diet of 0-groups caught near the surface. The results indicate that a food web of increasing complexity can develop during a relatively short time scale in late spring when the 0-group gadoids are pelagic. Generalised linear modelling revealed diel feeding patterns which were size-dependent and species-specific. In 2–3 cm cod, for example, peak feeding was during the period 1200–2000 h and high numbers of prey (mainly invertebrates) were consumed. The stomachs of the larger cod (3–5 cm) contained smaller numbers of mainly fish prey and peak feeding switched to 0000–0800 h. The piscivorous 0-groups were capable of eating relatively large prey items, providing the opportunity for larger individuals to eat younger or slower-growing individuals from the same year class. Whiting in particular were subject to substantial levels of cannibalism and inter-specific predation. Feeding interactions of this sort might be of sufficient magnitude to influence significantly the survival and recruitment of gadoids. High fecundity in whiting, coupled with multiple spawnings over a protracted period of time means that offspring from late spawnings could act as a food reserve for siblings from earlier spawnings. Such prey provide substantially more energy than the female invested in producing the original egg from which the prey was derived. This mechanism could prove to be an energy-efficient way of providing food reserves for the early offspring. It is also an efficient way of governing population size through density-dependent mortality.

Kristensen, Per Sand; Lassen, Hans

doi: 10.1006/jmsc.1996.0206pmid: N/A

Mussels (Mytilus edulis L.) smaller than the commercial size caught in Limfjorden, as in other areas, are typically discarded. However, during the period 1990 to 1993 these small mussels were returned, after sorting, to mussel beds for later harvest; a process defined as “relay”. This paper presents data from two commercial culture beds and from two smaller experimental beds established to study growth and mortality of these small mussel discards. The data were analysed by a yield-per-recruit model to calculate yields from such relays. This model was also used to predict the optimal time of harvest. The parameters utilized in the model were: (1) initial mortality due to harvesting, unshipping and sorting; (2) growth and mortality between relay and harvest; and, (3) the drained wet weight of a mussel of a given shell length. The initial mortality was estimated from observations of mussels damaged by harvesting, unshipping, sorting and exposure to desiccation on land in aquaria survival experiments. Growth and mortality rates after relay were estimated from diver investi-gations of the experimental beds. The relationship between the drained wet weight and the shell length was established based on samples from commercial landings in Limfjorden. The calculated harvest was compared to the observed yield from two commercial culture beds. The productivity in kg m−2 in a bed of relaid mussels was found to be 2–7 times higher than in natural mussel beds in Limfjorden.