ICES Journal of Marine Science: Journal du Conseil

doi: 10.1006/jmsc.1996.0001pmid: N/A

Jiang, W.; Jørgensen, T.

doi: 10.1006/jmsc.1996.0002pmid: N/A

An analysis of the diet of haddock in the Barents Sea based on 9500 stomachs collected between 1984 and 1991 is presented. The analysis was made on a quarterly basis for 10 cm fish length-classes. An overall 27% of the stomachs collected were empty and the average wet weight of the stomach contents was low for all size-classes (generally <10 g). By weight the prey categories Crustacea and Echinodermata made up an overall 60% of the diet, and fishes 20%. The ontogenetic change in the diet was small for haddock larger than 20 cm. Confidence limits for the estimated mean weight percentages were calculated using both theoretical variance estimators and bootstrapping. Precision was generally poor, with coefficients of variation typically in the range 0.4–0.6.

Cowan, James H.; Houde, Edward D.; Rose, Kenneth A.

doi: 10.1006/jmsc.1996.0003pmid: N/A

Twenty-day simulations of responses by a larval fish cohort were investigated with an individual-based model of predation by ctenophore, medusa, and planktivorous fish predators. Results indicate that the relationship between larval size and vulnerability to predation was generally dome-shaped for invertebrate predators and could be dome-shaped for fish predators if they foraged optimally by size, and depended upon attributes of both predators and larval fish prey. For the predators that did not forage optimally, cohort-specific mortality generally decreased as the mean length of larvae in a cohort increased, but bigger or faster-growing larvae within a cohort were not always most likely to survive. Until larvae grew through a “window” of vulnerability and reached a threshold length when susceptibility to the predators decreased more rapidly with larval length than encounter rate increased, mean length or growth rate of surviving larvae on each day was slightly lower, or not different from those that died in most of the simulations. After the threshold length was reached, predators began to catch smaller larvae, which resulted in larger survivors. The time necessary to grow through the window and reach the threshold length depended on growth rate of the larvae, size of the predators, and the variance structure of these parameters. These results indicate that size and growth rate of fish larvae are partially decoupled by the predation process and, ultimately, act differentially to determine cohort survival rate, although both may be most important after larvae have reached the threshold length. In these simulations, the threshold length was reached after a significant portion (56–99%) of total larval mortality had occurred; time to reach the threshold was generally shorter for the faster growing cohorts. Initially, both fast- and slow-growing individuals within a larval cohort differed little in size and, therefore, were nearly equally vulnerable to predation. However, reduced risk of predation occurred when all members of a cohort had reached the threshold length, which suggests that mean growth rate of individuals within a cohort, not their size, is probably the more important parameter affecting cumulative mortality, especially when the rate is high. We propose that characteristics of larval survivors may be more influenced by attributes of the predators to which they were exposed in early life, rather than by their initial status within a cohort with respect to length at hatching and potential growth rate.

Simmonds, E. J.; Fryer, R. J.

doi: 10.1006/jmsc.1996.0004pmid: N/A

This paper considers the design of acoustic surveys for estimating the mean abundance of spatially correlated populations. We examined how the choice of survey design affects the bias and precision of the sample mean as an estimator of mean abundance. Further, we investigated three different ways of estimating the error variance of the sample mean: the pooled within strata variance and two geostatistical variance estimators based on spherical and exponential models. First, we analysed four surveys to determine the spatial structure of the North Sea herring population. We developed forty different population models based on the observed amplitude and spatial distribution. We generated 1000 realizations of each model, each comprising 4000 potential transect means. Each realization was sub-sampled using eight different sampling strategies. From each realization and sampling strategy, we calculated the sample mean and three estimates of the variance of the sample mean.The simulations show that, for surfaces with local positive correlation, more precise estimates of the surface mean can be obtained using stratified random or systematic sampling than simple random sampling. The best strategy considered here was (a) a systematic survey with a geostatistical variance estimator, when the main objective is to obtain the most precise estimator of abundance, (b) a stratified random survey, with two transects per strata, and a pooled variance estimator, when an important objective of the survey is to obtain a good estimate of the variance of the abundance estimator.

Fossum, Petter

doi: 10.1006/jmsc.1996.0005pmid: N/A

Since 1987, the spawning stock of Norwegian spring-spawning (Nss) herring has increased from 0.5 to 2.5 million metric tonnes with recruitment of the 1983 year class to the spawning stock. During the same period the increase in herring larval production, resulting from an increase in the spawning stock biomass, has been followed by surveys on the Norwegian shelf in March and April. The purpose of the present study was to investigate whether the number of larvae can be related to spawning stock biomass and to look for density-dependent effects on gut-contents and larval growth. The strong signal from the 5-fold increase in spawning stock biomass was easily seen in the increased abundance of herring larvae since 1988. No density-dependent compensatory mechanisms related to growth were observed; however, a depensatory increase in dry weight was connected to the high abundance of post yolk sac larvae. This must be related to large scale spatial and temporal changes in prey abundance. Co-occurrence of herring larvae and their prey (match) could also be recognized in the early stages by the increased abundance of larvae beyond the yolk sac stage.

Heide-Jørgensen, M. P.; Reeves, Randall R.

doi: 10.1006/jmsc.1996.0006pmid: N/A

Aerial surveys of belugas, or white whales,Delphinapterus leucas, were conducted off West Greenland in March 1993 and 1994. These surveys were designed to permit comparisons with similar surveys in 1981, 1982, and 1991. Differences in methods between the surveys in the early 1980s and those in the 1990s should mean that the 1990s surveys were, if anything, more efficient than the 1980s surveys. Weighted linear regressions of strip-census estimates of the relative abundance of belugas showed a significant decline from 1981 to 1994. While annual variability in ice conditions and other unknown factors may have had some local or small-scale effects on the movements and distribution of belugas off West Greenland, it is unlikely that these effects would account for the apparent decline in relative abundance. The most reasonable conclusion is that the stock size has declined considerably during the past 13 years, perhaps by as much as 62%.

Winters, G. H.; Wheeler, J. P.

doi: 10.1006/jmsc.1996.0007pmid: N/A

A simple technique based on seasonal changes in gonad weights was used to describe key events in the spawning and maturation cycle of spring-spawning herring in the north-west Atlantic. Analysis showed that there were quite large inter-annual variations in the timing and duration of the maturation and spawning periods. The initial maturation process, which begins in the fall, is controlled mainly by phenotypic factors related to the size composition and condition of the adult population. The final maturation process, which begins in the spring and whose trajectory determines spawning times, is cued by January sea temperatures. The results contradict the general opinion that herring have a relatively fixed spawning season that is restricted to a brief 4–6 week period. Rather, the plasticity in spawning and maturation cycles of spring-spawning herring suggests that herring recruitment may not be a passive affair but an adaptive process in which Atlantic herring modify their reproductive activities to match expected environmental conditions during larval emergence.

Meisfjord, J.; Sundt, R. C.

doi: 10.1006/jmsc.1996.0008pmid: N/A

Liver and muscle tissue collected from harp seals (Phoca groenlandica) in the Newfoundland area were analysed by isoelectric focusing of five known variable enzyme systems. Allele frequencies were compared with corresponding frequencies previously recorded in north-east Atlantic samples. Muscle tissue collected from harp seals in the White Sea area and the Greenland Sea were analysed by DNA-fingerprinting. Samples were compared using band-sharing coefficients. Frequency distributions in AAT-2 and ME-1 were significantly different from those previously found in samples collected in the north-east Atlantic. Accordingly, it seems justified to divide the total stock of harp seals into one eastern and one western component. Highly variable profiles of Pal I digested genomic DNA were revealed using the human minisatellite 33.15 hybridization probe. Intra- and inter-population average band-sharing coefficients were uniformly low (range: 0.111–0.116). DNA-ingerprinting analyses did not reveal evidence for genetic differentiation between White Sea and Greenland Sea harp seals.

de Puelles, M. L. Fernández; Valdés, L.; Varela, M.; Alvarez-Ossorio, M. T.; Halliday, N.

doi: 10.1006/jmsc.1996.0009pmid: N/A

Diel variations in the vertical distribution of copepods were studied in the north coast of Spain and related to the physical and biological structure of the water column. Zooplankton samples were taken at three different stations during a 24 h period, in the proximity of a drogue buoy using a Longhurst Hardy Plankton Recorder (LHPR) sampling net, so that it is assumed that the same water mass was sampled continuously. Physical and other biological parameters were monitored by Conductivity Temperature Depth Recorder (CTD) profiles and vertical Undulating Oceanographic Recorder (UNDULATOR) dips for temperature, salinity, and chlorophyll a. Copepods were counted and classified into seven categories according to their size, and their distribution in the vertical plane was analysed using univariate and multivariate statistical techniques.Vertical distribution of copepods and their developmental stages were mainly determined by chlorophyll distribution and, when a subsurface chlorophyll maximum was present, small copepods and copepodite stages appeared aggregated in this layer. Nocturnal upward movements of the population and an ontogenetic component in the position and behaviour of the community were also important features that characterised the distribution of the community. Those patterns may explain a part of the observed variability of the abundances and distribution of the copepods in the water column. However, the water mass displacement in the 24 h period of study must also be considered as a main variability factor in the spatial and temporal distribution of the zooplankton populations.

Hansson, S.; Rudstam, L. G.; Kitchell, J. F.; Peppard, P. E.; Hildén, M.; Johnson, B. L.

doi: 10.1006/jmsc.1996.0010pmid: N/A

We compared four different methods for estimating predation rates by North Sea cod (Gadus morhua). Three estimates, based on gastric evacuation rates, came from an ICES multispecies working group and the fourth from a bioenergetics model. The bioenergetics model was developed from a review of literature on cod physiology. The three gastric evacuation rate models produced very different prey consumption estimates for small (<0.5 kg) and large (>2 kg) fish. For most size and age classes, the bioenergetics model predicted food consumption rates intermediate to those predicted by the gastric evacuation models.Using the standard ICES model and the average population abundance and age structure for 1974–1989, annual prey consumption by the North Sea cod population (age ≥1) was 840 kilotons. The other two evacuation rate models produced estimates of 1020 and 1640 kilotons, respectively. The bioenergetics model estimate was 1420 kilotons. The major differences between models were due to consumption rate estimates for younger age groups of cod.