ICES Journal of Marine Science: Journal du Conseil

Rijnsdorp, A. D.

doi: 10.1093/icesjms/48.3.253pmid: N/A

Variations in size-specific fecundity were studied over a period of seven years between 1977 and 1985 and fecundity-body length relationships were compared between three periods: 1900–1910, 1947–1949, and 1977–1985. Significant differences were observed between years and areas. The average fecundity for a 40-cm female was 12% less in the German Bight than in the southern North Sea, but the annual variability was higher; 15% and 7% respectively. Length-specific fecundity showed a significant positive correlation with the pre-spawning condition factor, but not with the somatic growth in the preceding year. Fecundity appears to have changed since the early 1900s and 1947–1949. In 30-cm females the present fecundity was about 30–100% higher compared to the first two periods, depending on the area. Fecundity of larger females (50 cm) was similar to that in 1900, but was 30–60% higher than in 1947–1949. The substantial reduction in fecundity in the period 1947–1949 contrasted with the much smaller change in ovary weight, suggesting not a change in the energy allocation over reproduction and somatic growth, but in the energy allocation over a small number of large eggs versus a large number of small eggs. Both length- and weight-specific fecundity decreased with age. The effect was largest in young fish. Weight-specific fecundity decreased by 3.9% when age increased from 4 to 5 years, by 1.7% when age increased from 10 to 11 years, and by 0.8% when age increased from 20 to 21 years. Significant differences in length-specific ovary weight were also observed between years and geographical areas, but the variability between years was less than in fecundity. Ovary growth was not synchronous between age groups, the younger age groups lagging behind by about one month. Egg weight calculated from ovary weight and fecundity was lower than egg weight measured from ripe running females, suggesting that ovary growth continues after the start of spawning in an individual plaice. The differences in fecundity and ovary weight between the three time periods are discussed in relation to the question whether these are a phenotypic response due to changes in the conditions for growth or to a change in the genetical composition of the population.

Fryer, R. J.

doi: 10.1093/icesjms/48.3.281pmid: N/A

The selectivity of the same fishing net can vary between hauls. Most selectivity models ignore this between-haul variation by pooling data over hauls. This approach is shown to result in misleadingly small parameter standard errors, which can lead to incorrect inferences about the differences between nets. A model is developed which incorporates between-haul variation and which provides realistic parameter standard errors. Methods of parameter estimation are described. The model is illustrated by a comparison of the selectivity of two nets.

Sampson, David B.

doi: 10.1093/icesjms/48.3.291pmid: N/A

Choosing a location for fishing is the major short-run decision made by the skipper of a fishing vessel. Because the spatial density of fish is not uniform everywhere, where a skipper decides to fish largely determines the size and value of his catch. For that to be a rational decision, the skipper must consider not only the catches he is likely to make at different locations but also the costs incurred in fishing at those locations. As a consequence the catch rates and the catch per unit effort observed in a fishery depend not just on fish stock abundance but also on economic factors such as wage rates and fish and fuel prices. This paper develops some simple theoretical models for examining a fisherman's selection of fishing location. The spatial distribution of the fish stock is reduced to a single dimension, distance from port, and it is assumed that fish density increases linearly with distance from port and that the relative densities remain constant regardless of the absolute level offish stock abundance. If a skipper operates his vessel further from port, he gains access to greater densities offish and higher instantaneous catch rates but uses more fuel and time for travel. A skipper can maximize his share of the fishing profits by operating his vessel at a particular distance from port. The skipper operates within constraints which determine the form of the revenue and cost functions. Two models are considered. In the first, the duration of a fishing trip is constrained by the size of the fish hold or by some other limit to the amount of fish that can be landed; each fishing trip continues until the hold is filled. In this case catch and revenue per trip are constant but the fishing costs vary non-linearly with distance from port. Here the catch per unit effort (c.p.u.e., catch per trip divided by time spent fishing per trip) is a non-linear function of the total biomass of the fish stock but the c.p.u.e. is independent of the price offish. In the second model, there is a time constraint; to fill the hold would take too much time. In this case each trip is of a fixed duration and catch and revenue per trip are quadratic functions of distance from port and operating costs vary linearly. Here c.p.u.e. is a linear function offish stock biomass and a non-linear function offish price.

Campana, Steven E.; Moksness, Erlend

doi: 10.1093/icesjms/48.3.303pmid: N/A

Known-age herring and cod larvae, reared in an outdoor mesocosm and the laboratory, respectively, were distributed to 18 study participants representing 10 different countries as part of an international otolith microstructure study. All details of rearing and sampling protocol were withheld from study participants, who were asked to estimate the age of multiple samples (aged 1–65 days) based only on the otolith microstructure. On average, age differences among samples were accurately estimated, while absolute ages (hatch dates) were underestimated. Otoliths from herring larvae less than 14 days old were the most difficult to interpret. Estimates of hatch date and measurements of the hatch check diameter differed significantly among readers, implying that differences in skill and/or increment interpretation among researchers could result in misleading differences among both collaborators and published results. The experience of the investigator and the degree of preparation of the larger otoliths were among the sources of the interinvestigator differences in ageing accuracy. With the possible exception of SEM, equipment type was not correlated with either accuracy or precision. The coefficient of variation (C.V.) of the age estimates decreased with larval age to a level of about 10–15%, and did not differ significantly among age readers. This study confirmed the utility of otolith microstructure examination for high-resolution age determination of young fish under quasi-natural conditions. Although the accuracy and precision levels characteristic of the herring otolith microstructure are probably somewhat lower than those of most other species, resolution of ± 1 day may be beyond the capabilities of the technique for a number of species.

Child, A. R.; Howell, B. R.; Houghton, R. G.

doi: 10.1093/icesjms/48.3.317pmid: N/A

Hourly sampling during five 24-h periods in the Thames estuary with a 2-m ring-net produced more than 14000 live sole eggs. The development stages of these eggs were determined within 30 min of capture and the results used to determine the time of spawning. There was a single peak of spawning each evening which ranged from 1615 h to 0100 h on different days and at different locations. Tidal phase appeared not to be a cause of the variation in timing.

Vranken, G.; Vanderhaeghen, R.; Heip, C.

doi: 10.1093/icesjms/48.3.325pmid: N/A

The toxicity of heavy metals (Cd, Co, Cu, Cr, Hg, Ni, Zn), acid-iron waste, pentachlorophenol, and γ-hexachlorocyclohexane on the marine nematode Monhystera disjuncta has been studied in laboratory assays. High levels of toxicants (< 1 mg/l) are necessary to cause acute effects on J2-larvae. The relative toxicity as measured by development retardation and reduction of fecundity is compared with threshold concentrations obtained from the acute tests. Fecundity is the most sensitive indicator. LC 50 values correlated significantly with the minimum concentrations of metals causing mortality and developmental inhibition. Ranking of toxicity based on fecundity does not correlate with ranking based on mortality or development rate. Monhystera disjuncta exhibits high resistance to pollutants compared with other benthic organisms.

McClatchie, Sam; Rakusa-Suszczewski, Stanislaw; Filcek, Krzysztof

doi: 10.1093/icesjms/48.3.335pmid: N/A

The mean size of adult and juvenile Euphausia superba in populations sampled during 1979–1986 in Admiralty Bay, South Shetland Islands, Antarctica shows a sinusoidal seasonal oscillation: mean krill size is reduced in winter. This has previously been interpreted as evidence of negative growth. Monthly total mortality was highest over winter, between July and October: there was notable interannual variability. We suggest that the winter-time decrease in mean krill size in Admiralty Bay results from predation reducing the numbers of larger, but less abundant krill, at a time when juvenile krill spawned the previous summer are recruiting to the population. This mechanism can explain the seasonal oscillation of mean krill size, without invoking negative growth.

Bakun, Andrew; Parrish, Richard H.

doi: 10.1093/icesjms/48.3.343pmid: N/A

A framework of comparative climatology of reproductive habitats of coastal pelagic fishes is extended to the anchovy inhabiting the shelf-sea ecosystem off Argentina, Uruguay, and southern Brazil. Maritime weather reports are summarized to yield distributions of wind stress, cloud cover, insolation, sea-surface temperature, wind mixing index, and Ekman transport. These distributions are considered together with other known aspects of the oceanography of the region and with seasonal and geographical aspects of reproductive activity. Over its extensive range, Engraulis anchoita spawns successfully within three different configurations of environmental processes affecting transport, water column stability, and trophic enrichment. One of these, incorporating a coastal indentation downstream from a coastal upwelling center, is very similar to the most common configuration characterizing spawning grounds of eastern ocean anchovy populations. The second, featuring interleaving water masses and upwelling at the continental shelf break, exhibits similarities to the reproductive habitat of the South African anchovy. The third, involving tidal mixing fronts, has been previously noted primarily in connection with herring of higher-latitude, shallow shelf-sea systems. The study adds support to the idea that similar fish populations in different regions must solve similar basic environmental problems and that various experiences of environmental effects in different populations, when viewed from a properly posed conceptual framework, can add up to a useful accumulation of insight.

Haug, Tore; Krøyer, Ari B.; Nilssen, Kjell T.; Ugland, Karl I.; Aspholm, Paul E.

doi: 10.1093/icesjms/48.3.363pmid: N/A

Since 1978 large numbers of harp seals (Phoca groenlandica) have invaded coastal areas of north Norway in winter and spring. In 1987 and 1988 dramatic increases occurred in both the magnitude and the geographic extent of these seal invasions. Sampling of stomach contents for food analyses and of teeth for age determinations was carried out on harp seals taken as by-catch in Norwegian gillnet fisheries in 1986 and 1988. It appears that the seal herds comprised both immature and mature animals, the latter being mainly males. The stomach analyses suggested that feeding was opportunistic, with a variety of fish (in particular the gadoid species cod, saithe, haddock, and Norway pout, and the pelagic shoaling species herring and capelin) being taken as prey. Prawns and squid were also consumed, but in considerably lower quantities than fish.

Ramster, John

doi: 10.1093/icesjms/48.3.373pmid: N/A