ICES Journal of Marine Science: Journal du Conseil

Cook, R. M.; Kunzlik, P. A.; Fryer, R. J.

doi: 10.1093/icesjms/48.1.1pmid: N/A

Sensitivity analysis is performed on the procedure for forecasting yield and biomass of North Sea cod (Gadus morhua L.) currently used by the International Council for the Exploration of the Sea (ICES). The yield forecast is most sensitive to estimates of recruitment during the forecast period, while spawning-stock biomass is most sensitive to mortality rates. Confidence intervals for forecast yield, spawning-stock biomass, and total stock biomass are calculated. These confidence intervals are based on variance estimates calculated by fitting a separable fishing mortality model to catch-at-age data. The calculated confidence intervals indicate that a useful improvement in the precision of the forecast can be achieved by using as much of the most recent recruitment data as possible to reduce the variance of the estimates of recruitment. In the case of North Sea cod this means delaying the assessment on which the forecast is based until late in the year. The analysis suggests that an autumn assessment would provide an adequate forecast of yield up to the third calendar year from the assessment and a forecast of spawning stock up to four years ahead.

Toresen, Reidar

doi: 10.1093/icesjms/48.1.15pmid: N/A

Predation on the eggs of demersal spawners has been reported several times, and evidence of haddock (Melanogrammus aeglefinus) feeding on capelin (Mallotus villosus) or herring eggs is a well-known phenomenon. A spawning ground where herring spawning has been observed over several years was selected as the study area, and investigations on the predation of eggs were carried out in three steps: (a) stomach sampling; (b) controlled experiments to find the rate of gastric emptying of herring eggs; and (c) bioacoustic surveys to study the distribution and abundance of predators. Haddock were found to be the main predator on eggs in the area. The total number of fertilized herring eggs being eaten in a period of 50 days was estimated to be about 2·7 × 1012, which was about 4% of the total egg production of the herring stock spawning in this area.

Hauser, J. W.; Sissenwine, M. P.

doi: 10.1093/icesjms/48.1.23pmid: N/A

A Monte Carlo simulation model was used to investigate uncertainty in estimates of the production of newly hatched larvae by a fish population when the estimates are derived from samples of larval abundance. The model simulates (1) hatching, growth, and mortality of a population of larvae; (2) a series of cruises which sample the population; (3) back-calculation to estimate the number hatched; and (4) comparison of estimates of the production of hatched larvae (LP). The model is applied for several combinations of (a) the time interval between cruises; (b) the coefficient of variation of larval concentration within a cruise (CVWC), which depends on sample size; and (c) for several other variables which characterize the simulated population. The coefficient of variation of estimates of the number of larvae produced (CVLP) increases sharply if the interval between cruises is greater than twice the standard deviation of the temporal distribution of the larval hatching. It is equally sensitive to CVWC. For example, doubling sample size by increasing the number of samples within cruises or by halving the interval between cruises has a similar effect. Of the random variables that characterize the population, CVLP is most sensitive to variability in growth rate. There is a bias in the estimate of larval production if mortality rate is length-dependent, or if estimates of growth rate used in calculations are incorrect. The simulations show that estimates of larval production are inherently uncertain.

Horwood, J. W.; Nicholson, M. D.

doi: 10.1093/icesjms/48.1.33pmid: N/A

A novel estimator of stock size from mark-recapture experiments is developed. Maximum likelihood estimators and their variances are derived assuming that catch is not known by age, relative recovery rates amongst different groups of collectors are different but constant over time within groups, catching effort is not known, and recruitment is either at an unknown rate proportional to the exploited stock size or else known relative to that in the first year. An example uses data from sole tagged in the Bristol Channel in 1959. The results indicate a stock size in 1960 substantially lower than previously estimated.

Hearn, William S.; Leigh, George M.; Beverton, Raymond J. H.

doi: 10.1093/icesjms/48.1.41pmid: N/A

The technique of attaching two tags, rather than one tag, to each fish in a tagging experiment is a well-established means of measuring the extent to which tags are shed, by observing the proportion of recaptured tagged fish that have retained only one tag. This commonly requires an assumption that tag-shedding is a random process which is unaffected by the presence of the other tag on the same fish. In theory, this makes possible an estimate of the unobservable number of caught fish that have shed both tags. This paper examines biases that may follow if data are pooled (sometimes unwittingly) from experiments with different tag-shedding rates. It is found that the bias in tag-shedding estimates, due to pooling, can be lowered markedly by reducing either the rate of shedding and/or variability between experiments in the tagging technique. The proportions of fish recovered in different experiments can sometimes be compared to support (or otherwise) the existence of bias. Southern bluefin tuna (Thunnus maccoyii) data sub-sets are analysed to illustrate the potential seriousness of tagshedding bias.

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

doi: 10.1093/icesjms/48.1.53pmid: N/A

An assessment of fertilization rate in a natural population of Dover sole, Solea solea (L.), is described. A preliminary study of the morphological changes of fertilized and unfertilized eggs from a captive stock was carried out to aid identification of sea-caught eggs. In addition, changes in specific gravity were measured to determine whether, and for how long, unfertilized eggs would be expected to be distributed similarly to their fertilized counterparts. During the field assessment, in which over 16 000 live eggs were examined, only 38 were judged to be unfertilized. This was estimated to represent a fertilization rate of over 99%. The paper also presents results from similar experiments on the eggs of plaice (Pleuronectes platessa L.).

Moksness, E.; Fossum, P.

doi: 10.1093/icesjms/48.1.61pmid: N/A

The otolith microstructure from North Sea autumn-spawned herring larvae and Norwegian spring-spawned herring larvae is described. The distance from the nucleus to the hatch check is on average 1.5 μm longer in the spring-spawned larvae (11.6 μm) than in the autumn-spawned (10.1 μm), and the average increment size is larger. The greatest difference is observed in the zone between 20 and 60 increments from the nucleus. This corresponds to a distance of 20 to 60 μm from the nucleus in the autumn-spawned herring larvae and of 28 to 132 μm in the spring-spawned larvae. A combination of distance to the hatch check and increment size in otolith from 30 to 60 μ from the nucleus could be a valuable method of distinguishing between the progeny of spring- and autumn-spawning herring.

Shepherd, J. G.

doi: 10.1093/icesjms/48.1.67pmid: N/A

A simple method for making short-term forecasts of catch and exploited biomass, based on a time-dependent stock-production model, is described. The method can be applied in various ways, depending on the nature of the data available. A short time series of landings estimates is required, and a reliable indicator of recruitment is highly desirable. The level of exploitation must also be estimated, but results are not usually very sensitive to this parameter. Given a good index of recruitment, the method is capable of giving satisfactorily precise catch forecasts even when the level of exploitation is high and recruitment is quite variable. The method can easily be implemented as a spreadsheet, and several examples are presented.

Addison, J. T.; Lovewell, S. R. J.

doi: 10.1093/icesjms/48.1.79pmid: N/A

The size composition of lobsters and crabs caught in the north Norfolk fishery differs markedly from that of other parts of the east coast of England. Previous studies indicated that substrate type and pot selectivity, in addition to the pattern of exploitation, account for the small individuals caught in Norfolk. Experimental fishing comparing fine-meshed small-ringed Norfolk pots and Yorkshire parlour pots was carried out off Cromer in north Norfolk and in Bridlington Bay, Yorkshire. Analysis of size composition of catches showed that both lobsters and crabs retained by the Norfolk pots were significantly smaller than those retained by the Yorkshire pots. The replication of the experiment in two fishing areas allows comparison of their size compositions when any effect of pot selectivity is equal in the two areas. The smaller size composition of lobsters caught in the Norfolk fishery was partially attributable to pot selectivity and partially due to real differences in size composition of the populations on the ground. For crabs there was little or no difference between the Norfolk and Bridlington Bay fisheries when the effect of pot selectivity was equal in the two areas. As the residual differences in lobster size composition were predominantly below the minimum legal size (and hence not subject to fishing mortality), the results suggest that on the east coast of England size compositions of lobsters are influenced by substrate type to a greater extent than those of crabs.

Nedreaas, Kjell; Nævdal, Gunnar

doi: 10.1093/icesjms/48.1.91pmid: N/A

The aim of this study was to develop and utilize electrophoretic methods for the taxonomic identification of Northeast Atlantic redfish (genus Sebastes). Morphological traits are insufficient for proper identification of these species, particularly for 0- and 1-group individuals. Haemoglobins and 17 enzymes in white muscle, heart, eye, and liver have been analysed. In adult specimens only a few of these enzymes are useful for separating between the species, so only 4 enzymes were analysed for the 0- and 1-group fish. The enzymes were identified by histochemical staining procedures. 0- and 1-group S. viviparus could be identified by diagnostic phenotypes of malate dehydrogenase (MDH) in all tissues, isocitrate dehydrogenase (IDH) in eye and muscle, and malic enzyme (ME) in liver. To identify 0- and 1-group specimens of S. mentella, blood samples are needed, as it is only the haemoglobin patterns that are diagnostic for this species. For positive identification of 0- and 1-group S. marinus, analysis of both haemoglobins and, e.g. MDH is necessary. However, in all three species, specimens less than 60 mm had not developed the adult haemoglobin pattern, thus making haemoglobin of no value in the classification of smaller 0-group specimens, and the separation of S. marinus and S. mentella specimens less than 60 mm impossible. The three species showed different frequency distributions of IDH (liver) and ME (muscle and liver) phenotypes, thus making it possible to estimate the most likely proportion of individuals from each species when they occur in a mixture.