ICES Journal of Marine Science: Journal du Conseil

Marchal, Paul; Horwood, Joseph

doi: 10.1006/jmsc.1995.0076pmid: N/A

ICES has focused attention on minimum biologically acceptable levels (MBALs) as a basis for ensuring the safety of stocks and providing advice for management. Simultaneously the European Commission has proposed the use of multi-annual management strategies. This study models the effect of TACs, set constant for different periods of years, on the levels and variability of spawning stock biomass (SSB) and yield. The Celtic Sea cod is used as an example. It is shown that, even with annual updating of TACs, target SSBs have to be substantially greater, possibly more than seven times the MBAL, to avoid the risk of stocks falling below the critical level. Even if the stock is allowed to fall below the MBAL for 5% of the time, the target stock sizes need to be 3–10 times the MBAL if TACs are updated at 2–10 year intervals. As the period of stable catches is increased the problems of safeguarding the stock from falling below the MBAL also increase, and stability has to be paid for by lower target fishing mortalities. Specifically for the Celtic Sea cod, a balance of high yield and safety could be achieved with average catches of 7900 t and a target spawning stock biomass of 30 000 t.

van der Meer, Jaap; Leopold, Mardik F.

doi: 10.1006/jmsc.1995.0077pmid: N/A

European storm petrels were counted on criss-cross ship transects on the continental shelf between Ireland and Spain. No element of randomisation was involved in the selection of the sampled area. A geostatistical model which explicitly considers the spatial autocorrelation between observations from transect segments, was therefore used to predict population size in the study area. The model also provides standard errors of the prediction. Although some of the underlying assumptions of the model were questionable, several alternative approaches yielded almost similar predictions, suggesting model robustness. The predictionof the total number of European storm petrels present in the study area is equivalent to 255 000 breeding pairs. This is much higher than previous estimates of the total breeding population of the region, ranging from 50 000 to 100 000 pairs.

Sparholt, Henrik

doi: 10.1006/jmsc.1995.0078pmid: N/A

Cod cannibalism in the central Baltic is estimated by the Multi-Species VPA (MSVPA) Model. By using the corresponding Multi-Species Forward (MSFOR) Projection Model, Ricker curve relationships were found for the relationship between cod spawning stock biomass (SSB) and recruitment of 1-, 2-, and 3-year-old cod, assuming that recruitment of 0-year-old cod was proportional to the SSB. It is argued that a proportional relationship between 0-year-old cod and SSB was the most simple and biological reasonable assumption for central Baltic cod, unless the SSB was extremely large. The SSB level that gave the highest number of recruits at age three was 500 000 t, and this value was independent of the assumed proportionality factor between recruitment of 0-year-old cod and SSB. The proportionality factor changed from year to year and this was postulated to be environmentally driven. Independent of the environmental conditions in a given year, the maximum recruitment of age three cod was obtained at SSB values of 500 000 t. This finding has important fisheries management implications. Cannibalism is an important self-regulatory mechanism in the central Baltic cod stock.

MacLennan, David N.

doi: 10.1006/jmsc.1995.0097pmid: N/A

For constant fishing effort, the variation of yield is generally driven by largely unpredictable changes in annual recruitment. Given the statistics of recruitment fluctuations, yield variability depends on the level of effort and the selection parameters of the fishing gear. This paper examines the significance of the latter, with particular reference to the shape of the selection ogive.An age-structured population model was used to develop the theory of yield variation. Gears are selective on fish length and the model takes account of stochastic growth in each cohort. Data from trawl surveys in the North Sea suggest that the length-at-age distribution is Gaussian with a standard deviation which increases linearly with age.The model is applied to the North Sea haddock fishery using data from quarterly trawl surveys in 1991. The catch analysis is performed in quarterly steps to take account of the within-year growth of young fish. The results show that yield is least variable when the selection range is of the order of 50% of the retention length. Thus “knife edge” selection (zero selection range) is not the optimal harvest strategy when the objective is to minimise yield variability.

Abend, Alan G.; Smith, Tim D.

doi: 10.1006/jmsc.1995.0080pmid: N/A

Naturally occurring nitrogen stable isotopes found in tissues have been shown to differ among animals feeding on various prey species and at different trophic levels. The ratio of 15N to 14N, expressed relative to atmospheric nitrogen, is compared for four types of tissues collected from long-finned pilot whales from three locations in the North Atlantic. The tissues were skin, muscle, blubber, and teeth, arranged in decreasing rates of biological turnover. The tissues with fastest turnover rates reflect the most recent diets. The ratios differed among tissues, reflecting possible changes in diet over time, and within tissues among the three geographic locations reflecting different dietary histories. Significant differences in ratios from skin, muscle, and blubber between animals from the eastern and western Atlantic suggest the whales are not feeding at the same trophic level. That is, there are more predator-prey relationships separating western Atlantic pilot whales and their primary production. Differences in the isotope rations for blubber, a lower turnover rate tissue, between whales from Cape Cod and the Mid-Atlantic Bight suggest that whales from these areas were feeding in different areas over the longer term. Different patterns of variability in nitrogen isotope ratios from pilot whale teeth among the three regions suggest potentially more complex, longer term movement patterns.

Christensen, Steen

doi: 10.1006/jmsc.1995.0081pmid: N/A

The potential impact of reduced mortality of codend escapees was investigated by a dynamic pool model applying constant fishing effort on a varying stock. The model is applied to data from the Greenland offshore shrimp (Pandalus borealis) fishery.The study indicates that the potential impact of an increase in mesh size depends heavily on the present level of mortality of the codend escapees, Fε. Simulations show that the decrease in long term equilibrium catches following a mesh size increase from 43 mm to 55 mm is proportional to Fε. The economic break-even point is at a mortality rate of 50% per year.

Sorokin, Yu. I.; Giovanardi, O.

doi: 10.1006/jmsc.1995.0082pmid: N/A

Parameters of feeding and food balance in the Manila clam Tapes philippinarum and in the carpet shell Tapes decussatus were estimated using 14C-labelled algae and bacteria as food. The Manila clam has a wide feeding spectrum, consuming and digesting various kinds of algae, bacteria, and rotifers. The optimal concentrations of algae (Nitzschia, Chlorella) were found to be around 8–9 mg l−1 (wet weight) and those of bacteria, 4.5–5 mg l−1. The threshold concentration of food (where assimilation rate=respiration rate) was 1.6–1.7 mg l−1 of algae and 1.5 mg l−1 of bacteria. The filtration rate fell to between 3 and 6 l h−1 g−1 flesh carbon, or 0.1–0.9 l h−1 per specimen. The food ration per day was 3–5% of body weight, or 30–50 mg C g−1 of flesh carbon with optimal food concentrations. The assimilation of green algae and diatoms consumed as food was 50–60%, and the efficiency of the use of assimilated food for growth (k2) was 0.36–0.49, depending on the kind of food (algae). The data obtained were used to evaluate trophic conditions for farming clams in the Lagoon of Venice, by comparing food requirements with the real food concentrations found in the lagoon.

de Barros, Pedro; Holst, Jens Christian

doi: 10.1006/jmsc.1995.0083pmid: N/A

The Norwegian spring-spawning herring stock is characterised by a highly variable recruitment. The juveniles are found from the fjords of west Norway to the north-eastern Barents Sea, in a nursery area covering both temperate and boreal waters. High recruitment to the spawning stock is associated with year-classes that are mostly distributed in the Barents Sea as juveniles. The growth, maturation, and recruitment patterns of the fish spending their first years of life in the Barents Sea are distinct from those of the herring further south along the Norwegian west coast.For the management of this stock, it is important to identify the fish from the different nursery areas as they recruit to the fishery and the spawning stock. A method to discriminate the fish from the Barents Sea and the Norwegian west coast nursery areas is based on the width of the scale annuli. The success rate of the derived classification rule is estimated to be about 90%. The proportion of fish from each area in a sample can be estimated with a precision of ±4%.