Comparative fishing experiments by research trawlers for cod and haddock in the North SeaEhrich, Siegfried
doi: 10.1093/icesjms/47.3.275pmid: N/A
For 10 days in June 1986 a comparative fishing experiment was carrried out in a small area of 240 nm2 in the northern North Sea. Three ship-gear combinations were compared: (1) the FRV “Walther Herwig”. with the French “GOV trawl (36/47 m)” but with a rubber disc groundrope; (2) the FRV “Anton Dohrn”, with the “GOV trawl (36/47 m)”; and (3) FRV “Anton Dohrn”, with the German “180-foot herring trawl”, in each case rigged with a heavy bobbin groundgear. A significant difference in the efficiency of capture of haddock was found only with the different nets, higher values occurring with the “180-foot herring trawl” (conversion factor = 1.45). The combined vessel and groundgear effect on the haddock catches was statistically insignificant. In contrast to haddock, the capture efficiency for small cod was strongly dependent on the groundgear. Compared with the rubber disc groundrope, about 85% of cod smaller than 32 cm escaped between the heavy bobbins under the fishing line. For bigger cod (32cm), the capture efficiency of the “GOV trawl” is significantly higher (conversion factor = 2.4) than that of the “180-foot herring trawl”.
Multiplicative modelling of catch-at-age data, and its application to catch forecastsShepherd, J. G.; Nicholson, M. D.
doi: 10.1093/icesjms/47.3.284pmid: N/A
A natural interpretation of fish catch-at-age data leads to an approximate multiplicative model with three factors: year, age, and year class. These factors are however, interrelated (year class = year - age) and estimates of the parameters of such a model are not unique. Specific solutions may be obtained by imposing biologically meaningful constraints on the parameters: for example, by specifying the trend in the year effect. The choice of error distribution and its parameters is also important in fitting such models. Examination of the sampling procedures used in data collection suggests a simple approximate formulation for the error variance of the log-transformed data. The model may be fitted by standard least-squares methods, or by a simpler calculation based on log-catch ratios. The model may be applied to any coherent set of catch-at-age data, representing the total international catch, that of a single fleet, or a research survey. The method is independent of VPA, but conceptually closely related to separable VPA. It may be used to estimate the steadystate age composition (i.e. a corrected catch curve), which is a required starting point for some assessment procedures, and also to estimate relative year-class strength for all year classes represented in the data, even those only present as older ages in early years. The fitted parameters may also be used as the basis of a simple forecast of catch-at-age for the data set to which it has been fitted.
The biomass of starry ray (Raja radiata) in the North SeaSparholt, Henrik; Vinther, Morten
doi: 10.1093/icesjms/47.3.295pmid: N/A
A rough estimate of the biomass of starry ray in the North Sea is given based on three different methods. The first relates the catch rate of starry ray in a bottomtrawl survey to the biomass by comparing the catch rate of starry ray with that of plaice, whose biomass is known from standard assessment methods. The second method is based on the same principles but uses the relative catch rates of starry ray and plaice in a beam-trawl survey. The third method relates mortality estimates from catch curves to estimated commercial catches and by-catches. The biomass estimates from these methods are in reasonable agreement with each other, around 100 000 t as a mean over 1977–1988. During this period the year-to-year variation in biomass was no greater than twofold.
Otolith measurement as a method of identifying factors affecting first-year growth and stock separation of mackerel (Scomber scombrus L.)Dawson, Wendy A.
doi: 10.1093/icesjms/47.3.303pmid: N/A
The amount of growth in the first year can be measured from the otolith (L1). This difference in first-year growth has been associated with the difference in the peak spawning times between the North Sea and Western mackerel stocks. These differences in growth are analysed, and their suitability considered for stock separation when catches are taken in areas where the two stocks are known to be mixing. Firstyear growth has also been shown to vary between year classes. While the variation for both stocks follows the same trends, the difference in growth between Western and North Sea fish has not always been in the same direction. Prior to 1974. firstyear growth was significantly greater for the Western stock than for the North Sea stock. Since 1974 the amount of growth in the first year has been the same or larger for the North Sea stock. The changes in growth of the North Sea fish have been linked with density dependence.
An approach to better management: the North Sea haddockHorwood, Joseph
doi: 10.1093/icesjms/47.3.318pmid: N/A
This study argues for a focus of attention on bioeconomic objectives in the management of fisheries of the Northeast Atlantic. It indicates an approach towards identifying agreed dynamic and bioeconomic relationships of a fishery. Once such functions are defined, a near-optimal control is developed that allows for regulation of a fishery towards and about an optimum. It accommodates variation in recruitment and in estimates of stock size. A demanding example is presented based upon the North Sea haddock, which at present is much below any optimum level.
Allozyme data and scallop stock identificationBeaumont, A. R.
doi: 10.1093/icesjms/47.3.333pmid: N/A
Scallop fishery management models require information about the degree of genetic isolation of “stocks” and recruitment into beds within those stocks. The principles behind the gathering of allozyme data are presented in this paper, and evidence is put forward to show that these data have proved useful for demonstrating significant population differentiation in the queen scallop (Aequipecten (Chlamys) opercularis). However, a survey of populations of Pecten maximus has not revealed similar heterogeneity and, furthermore, knowledge on scallop recruitment at a local level cannot usually be deduced from allozyme data. Data from electrophoretic surveys of other scallop species are briefly discussed, and it is concluded that although allozyme data still have a role to play, they are best included within a multidisciplinary, biological/oceanographical approach to stock recruitment and identification.
Food and feeding ecology of five gadoid larvae in the northern North SeaEconomou, Alcibiades N.
doi: 10.1093/icesjms/47.3.339pmid: N/A
The larval diets of five gadoids in the northern North Sea in May in both 1978 and 1979 are described and compared. Cod, Norway pout, saithe, and whiting larvae consumed similar types of prey and showed a general tendency to select increasingly larger prey with increasing body size. The haddock larvae exploited a wider range of prey types, and at comparable lengths they consumed smaller and slower moving organisms than did the other gadoid larvae. The diets of sandeels and long rough dab which were abundant in the area were also examined. The species-specific selectivity patterns with respect to size and mobility of prey fell into two categories: those dictated by the basic body morphology, and those determined by behavioural factors, which were intimately linked to adult behavioural patterns. Competition for food was potentially possible between late larvae, but it could not be identified as a factor causing shifts in dietary characteristics.
Reproductive variability in North Sea plaice, sole, and cod1Rijnsdorp, A. D.; Daan, N.; van Beek, F. A.; Heessen, H. J. L.
doi: 10.1093/icesjms/47.3.352pmid: N/A
The between-year variability in reproductive potential of the stocks of North Sea plaice, sole, and cod is investigated using time series of growth, maturation, and fecundity. Special attention is given to possible density-dependent effects. The growth rate of both plaice and sole increased in the 1960s. Evidence of density-dependent effects was observed only in the somatic growth of juvenile plaice and in the condition factor of sole. However, these results can also be explained by concurrent time trends. The age, but not length, of plaice at first maturity was correlated with juvenile growth. Fecundity of cod and ovary weight of sole were positively correlated with spawning biomass, whereas a weak positive correlation between fecundity and growth was observed in plaice. In general, there was very little evidence that variability in the reproductive parameters of these species is related to population density. The data suggest that conditions for growth vary substantially between years, but are independent of stock size at the present level of exploitation. Nevertheless, the observed changes in growth, maturation, and fecundity appear to have compensated for the losses in total egg production due to increased exploitation by about 25% for all species. It is suggested that effects of reproductive variability which are related to the age composition of the spawning population, and which may affect the quality of eggs (e.g. proportion of recruit spawners), could be more important than the density-dependent effects.