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doi: 10.1093/icesjms/39.3.211pmid: N/A
A form of the log-linear model for modelling catch per unit effort (c.p.u.e.) is described which treats the year category as an ANOVA classification. This model allows the calculation of two standardized relative abundance indices: 1) direct interpretation of regression coefficients for the year category, and 2) the usual estimates of c.p.u.e. calculated using standardized effort (adjusted c.p.u.e.). The regression coefficient estimates have advantages in theoretical simplicity and computational convenience.The model is applied to two trawl-fished populations of Pacific ocean perch (Sebastes alutus) in the northeast Pacific. Catch and effort data from 1953 to 1976 were used to calculate relative abundance indices standardized for gross tonnage, echo sounders with paper recorders, and roller (bobbin) gear. Stock conditions for the two populations have been historically different, and this factor is shown to interact with the impact of technological improvements. In the Queen Charlotte Sound region, where stocks have been and remain in relatively good condition, technological improvements had little measurable effect on catch rates. However, in the west Vancouver Island region, where stocks have been smaller and are now depleted, these same technological improvements (echo sounders with paper recorders and roller gear) doubled the estimated efficiency of vessels.Due principally to catches by large Japanese and Soviet trawlers, relative abundance indices in both the Charlotte and Vancouver regions registered a sharp decline (50% and greater) in 1965 to 1968. This decline was particularly severe in the Vancouver region. In the Charlotte region, relative abundance measured by regression coefficient estimates and adjusted c.p.u.e.s were nearly identical; but in the Vancouver region, these estimates differed. Considering the methods used in calculation, it was concluded that this difference was probably due to chance events rather than systematic bias.The use of a qualifying criterion for selecting data (deleting data for which the catch is below some fixed percentage of the study species) can greatly affect estimates of relative abundance in multi-species fisheries. In the Charlotte region, a qualifying percentage of 25 % had little effect on relative abundance estimates. But for the Vancouver region, a qualifying percentage of 25 % significantly lessened the estimated decline in abundance.In summary, it appears that there is generally an interaction between the effects of technological improvements in a fishing fleet and the condition of fish stocks. It appears that technological improvements sometimes have their greatest measurable impact on fully utilized or depleted stocks. Also, the selection of qualification levels will sometimes be critical when measuring large declines in stock abundance.
Behrens, William J.; Duedall, Iver W.
doi: 10.1093/icesjms/39.3.219pmid: N/A
Simultaneous analyses of changes in dry body weight, heavy metal concentrations, and total body burden of heavy metals in Mercenaria indicate that temporal variations in metal levels are associated with biological processes of the organism. Changes in Ni, Pb, and Cu levels in hard clams are related to the spawning cycle while Cd, Cr, and Zn levels are not. The interpretation of fluctuations in metal concentrations may be misleading unless considered with respect to seasonal variations in body weight.
Behrens, William J.; Duedall, Iver W.
doi: 10.1093/icesjms/39.3.223pmid: N/A
An area in Great South Bay, NY which has been closed to shellfishing on the basis of coliform bacteria standards was shown to have elevated levels of heavy metals in hard clams, Mercenaria mercenaria, and sediments. Clams from this area were transplanted into the central portion of the Bay which is open to shellfishing. The behaviour of heavy metals in the transplanted clams was primarily affected by long-term trends based on seasonal fluctuations in the organisms. While these clams were being depurated of bacteria, no depuration of any heavy metal analysed (Cd, Cr, Cu, Ni, Pb, and Zn) was noted over the 50-day duration of this study. On the contrary, significant increases in the total body content of Cd, Ni, and Pb occurred. Resulting Cd and Pb levels in hard clams were not elevated above natural levels found in the transplantation area, but Ni levels were approximately 56% higher. This transplantation therefore introduced hard clams with significantly higher levels of Ni into the harvestable resource.
Shepherd, J. G.; Garrod, D. J.
doi: 10.1093/icesjms/39.3.231pmid: N/A
Projecting the effects of changes in the availability of fish resources on the operations of a fishing fleet is a large and complicated allocation problem. We have found that linear programming is unsuitable for modelling such problems, because the solutions are inherently extreme, sparse, and ruthless, whereas the real fishing industry has substantial inertia. There are powerful socio-economic reasons why it cannot and should not depart too rapidly or radically from the status quo.We have developed the method of cautious non-linear optimization to handle the problem. This ascribes non-linear penalties to deviations from a reference solution, and any constraints which fail to be observed, and minimizes the resulting compound objective function using the conjugate gradient method. A fairly large problem (1500 variables) can be handled on a moderately sized computer.Excellent results have been obtained. The model is able to reproduce an actual situation when given the appropriate data, and can model changes which have happened during the past few years reasonably accurately. We believe that the uncertainties in the results are caused almost entirely by uncertainties in the assumptions and data used, and hardly at all by the modelling technique. A range of problems can be studied by varying the weights ascribed to the various terms of the objective function, and the results obtained seem to be meaningful even at a highly disaggregated level. We consider that the method is most powerful, and would be useful in many problems where a large and complex system with substantial inertia has to be modelled.
Shelton, P. M. J.; Shelton, R. G. J.; Richards, P. R.
doi: 10.1093/icesjms/39.3.239pmid: N/A
In some Arthropoda it is possible to determine the stadium (moult stage) which an individual has attained by counting the number of ommatidia in the eye. This paper presents the results of an experiment using cultured juvenile lobsters of known moult history to see whether the same method can be applied to the European lobster, Homarus gammarus. It was found that the eye grows isometrically with respect to the carapace and that counts of ommatidia are therefore an unreliable guide to moult stage. Over the restricted range of this experiment (moult stages 6 to 12) it was found that counts of ommatidia indicated moult stage with an accuracy of two stadia only. It is concluded that no other surface feature is likely to provide an unequivocal indication of the stadium.
Hislop, J. R. G.; Shanks, A. M.
doi: 10.1093/icesjms/39.3.244pmid: N/A
The reproductive biology of haddock, Melanogrammus aeglefinus (L.) from the northern North Sea was investigated during the years 1976 to 1979. There was no evidence that the observed relationship between fecundity and length, fecundity = 0·903 × length3·42, differed from that given by Raitt (1933) for the period 1926 to 1930. The relationship between fecundity and gutted weight was calculated as F = 237 × weight1·13. The percentages of mature fish in age groups I, II, III, IV, V+ were 2, 46, 93, 100. 92 and 0, 30, 80, 94, 100 for males and females respectively. Compared with the period 1926 to 1930 there has been an increase in the percentage of mature 2-year-old females and a reduction in the percentage of mature 2-year-old males. The length at which 50% of the fish were mature was estimated to be 27·5 cm for males and 31·5 cm for females. The fecundity of haddock infected with Lernaeocera branchialis was 21% lower than that of uninfected fish. Although infected fish had significantly lower condition factors than uninfected fish, the difference in fecundity at length between the two groups was mainly attributable to the much lower production of eggs per unit of gutted weight by the infected haddock.
doi: 10.1093/icesjms/39.3.252pmid: N/A
Compression tests showed that high levels of Polydora ciliata infestation tended to weaken the shells of Mytilus edulis. A predation experiment indicated that the heavily infested, weak-shelled mussels may be more vulnerable to the predatory activities of the crab, Cancer pagurus.
doi: 10.1093/icesjms/39.3.256pmid: N/A
Estimates of the rates of tag shedding by southern bluefin tuna, Thunnus maccoyii, are obtained using data from a long-term double-tagging experiment. It is found that the standard assumption of a constant tag-shedding rate is inappropriate for these data, and generalized models are developed to allow for a varying rate of tag shedding.
doi: 10.1093/icesjms/39.3.261pmid: N/A
Three surveys were carried out in April 1977 in the Southern Bight of the North Sea to investigate variance in plankton, environmental properties, and their interrelationships. Fluorescence, particle-size groups, and environmental variables were monitored over 100 km transects. Fourier techniques were used in the analyses. Large amplitude small-scale variability was absent in fluorescence but patches of 25 km length were observed in the 30 to 80 μm particle-size range. Fluorescence data indicated a spatial structure similar to that predicted by Denman and Platt (1976) and Fourier spectra of fluorescence, environmental parameters, and particle-size groups were different from each other. The spectra of the particle-size groups were flat or else had a small slope. Coherences were generally high amongst all variables but coherences between specific biological and environmental variables occurred at different space scales suggesting that the biological system may be forced at different space scales by different environmental components. Some substantial decreases in pH were noted.
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