ICES Journal of Marine Science: Journal du Conseil

Kahru, M.; Aitsam, A.

doi: 10.1093/icesjms/42.2.111pmid: N/A

Within the time and space scales of quasi-synoptic areal surveys (∼1 day, ∼50 km) thefluorescence, measured by an in situ profiler, is shown to be a reliable measure of the chlorophyll a concentration, although significant areal and temporal differences in the fluorescence yield were observed. Analysis of variance is used to partition the total chlorophyll variance into effects due to differences between large-scale areas in different basins, to time differences between surveys, and to synoptic-scale (∼10 km) and fine-scale (∼100 m) spatial variability. It is shown that the large-scale areal differences in chlorophyll concentration are inevitably missed, being over-shadowed by the other components of variance. The dominant sources of variance are due to the time and to the synoptic-scale space variability. With respect to the variance structure, the total water column chlorophyll is preferable for monitoring purposes. The results point up weaknesses in the conventional practice of monitoring a highly variable parameter via a sparse gridafew times a year.

Rothschild, B. J.; Mullen, A. J.

doi: 10.1093/icesjms/42.2.116pmid: N/A

This paper considers the information content of stock-and-recruitment data and its non-parametric classification. Stock-and-recruitment observations representing several species have been classified as states according to whether they are low stock/low recruitment; low stock/high recruitment; high stock/high recruitment; or high stock/low recruitment. The classification makes it possible to calculate transitions among the states, steady-state probabilities, and first-passage times. If there is reason to believe that there is a “good” stock-and-recruitment relationship, then the parametric approach is better than the non-parametric approach, but if the relationship is poor the non-parametric approach may provide more useful management advice, in addition to taking into accountthe temporal relation among stock-and-recruitment points. The paper emphasizes that a major problem with either the parametric or the non-parametricapproach is that inferencesfor the future from these analyses depend upon the assumptionthat the unidentified mechanisms that caused recruitment to vary in the past will continue to behave in the same way in the future.

Brander, K.; Palmer, D.

doi: 10.1093/icesjms/42.2.125pmid: N/A

The growth rate of Raia clavata is estimated by examination of length-frequency modes in samples of fish caught by trawling off the North Wales coast over the period 1976–1982. Only the size of the youngest ages can be established in this way. The results differ from those presented recently by Ryland and Ajayi (1984), and wediscuss the two sets in relation to the information on time of spawning and incubation period for this species.

Pope, J. G.; Shepherd, J. G.

doi: 10.1093/icesjms/42.2.129pmid: N/A

Various methods for tuning virtual population analyses (VPAs) using effort data have been tested on simulated data representing several sorts of fish stocks and fisheries.When there are no systematic changes in catchability, all the methods tested work reasonably well even in the presence of substantial random noise (coefficients of variation of the order of 50 %) in both the catch numbers and effort data. However, systematic trends in catchability may cause gross errors or failure of all current methods except the Rho method, and even this may fail if the trends are sufficiently strong. A new method is proposed which has a low bias and variability even in the presence of strong trends and noise, and which should be fairly widely applicable.

Patterson, K. R.

doi: 10.1093/icesjms/42.2.152pmid: N/A

Whiting (Merlangius merlangus) were trawled from a muddy sand trawling ground in the Irish Sea during 1981 and 1982. They had been feeding mainly on Norway pout (Trisopierus esmarkii), sprat (Spratlus sprattus), sandeels (Ammodytes marinus), herring (Clupea harengus), and cuttlefish (Sepiola atlantica). There was considerable seasonal variation in both the quantity and the type of food eaten. Yearly mean foodintake rate was estimated at 1·42, 3·35, and 7·25 grams fresh weight per day for one-, two-, and three-year-old fish respectively. Gross conversion efficiency during the second, third, and fourth years of life was calculated to be 0·14. Comparison with laboratory data suggeststhat whiting may use between 10 and 45 % of available metabolites for locomotion. Predation on herring seemed to be important, and may have accounted for 45 % of the variability in the numbers of recruits to the local stock.

Houghton, R. G.; Last, J. M.; Bromley, P. J.

doi: 10.1093/icesjms/42.2.162pmid: N/A

The eggs of soles (Solea solea (L.)) spawning in captivity were collected using an airlift egg filter. Three spawning females weighing 2·03 kg produced 954 000 eggs in 22 batches over 47 days. Total fecundity was 471 eggs/g whole weight. Only 51% of the eggs were fertilized. Egg diameter decreased from 1·4 to 1·2 mm during the spawning period.

Farebrother, R. W.

doi: 10.1093/icesjms/42.2.166pmid: N/A

In this paper we examine Beverton's log-linear model for the recapture times of a single release of tagged fish. We find (1) that there is a minor error in Paulik's formulas for the generalized least-squares estimates of the parameters of the model, (2) that the generalized least-squares estimates may be obtained by applying the method of weighted least squares with suitably chosen weights, and (3) that the generalized least-squares estimate of an additional parameter may be used as the basis of a test for model misspecification.

Cook, R. M.; Armstrong, D. W.

doi: 10.1093/icesjms/42.2.171pmid: N/A

The catchability coefficient, Q, of cod, haddock, and whiting associated with the Scottish seine-net fleet is examined. Cod and haddock show increasing catchability for the period 1963–1979. The rise in Q is related to the spatial distribution of the fleet as vessels increasingly fish in areas of higher fish abundance. Additionally, for haddock, catchability is related to year-class strength. The utility of catchability trends in the prediction of fishing mortality is discussed.

Colebrook, J. M.

doi: 10.1093/icesjms/42.2.179pmid: N/A

Sea surface temperatures for the North Sea for the period 1948 to 1983 do not show any clear trend but are rather dominated by apparently periodic fluctuations on relativelyshort time scales, at 9–11 years and, from February to June, also at 3–3·5 years. The short period events may be related to fluctuations on a similar time scale in the frequency of westerly weather over the United Kingdom and also to fluctuations in the abundance of the zooplankton. The relationship between temperature and plankton is negative, with the temperature leading the plankton by about a quarter of a wavelength. The problem of interpreting this relationship is investigated using a simple simulation model.

McCurdy, William J.

doi: 10.1093/icesjms/42.2.186pmid: N/A