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doi: 10.1093/icesjms/37.3.199pmid: N/A
In the common models of stock assessment, fish populations are stable and minor disturbing forces (e.g. small increments in fishing effort) cause only minor changes in the equilibrium position. Clark's (1974) conclusion, based on a “salmon” type of fish, that if there is a depensatory part of the stock/recruit relation, there will be an unstable situation, is extended to all types of population. A similar situation, in which a small increment in fishing effort can result in a collapse of the stock can also occur if (as seems to happen in some pelagic fisheries) the catchability coefficient, q, can increase with decrease in fish stock. This potential for collapse can be serious in fishery management unless there is adequate provision for monitoring the stocks, and for rapid implementation of control measures as they become necessary.
Szekielda, Karl-Heinz; Suszkowski, Dennis J.; Tabor, Paul S.
doi: 10.1093/icesjms/37.3.205pmid: N/A
The upwelling off the NW coast of Africa in the vicinity of Cap Blanc was studied in February-March 1974 from aircraft and in September 1973 from Skylab. The aircraft study was designed to determine the effectiveness of a differential radiometer in quantifying surface chlorophyll concentrations. Photographic images of the S190A Multispectral Camera and the S190B Earth Terrain Camera from Skylab were used to study distributional patterns of suspended material and to locate ocean colour boundaries. The thermal channel of the S192 Multispectral Scanner was used to map sea-surface temperature distributions off Cap Blanc. Correlating ocean colour changes with temperature gradients is an effective method of qualitatively estimating biological productivity in the upwelling region off Africa.
doi: 10.1093/icesjms/37.3.214pmid: N/A
The study is based on the examination of 568 individuals (255 males, 313 females) caught in Tunisian waters in 1973–75. Pteromylaeus bovinus — the Bull ray — is a voracious species and the stomachs of over 70% of the individuals examined contained food. But the intake of adult females is reduced in summer when the eggs are shed. As with other bottom living elasmobranchs, benthic invertebrates and fish appear to be favoured as food. Teleosts form an important part of the diet in both young and older fish, and this somewhat exceptional feature is associated with the high growth rate of the Bull ray and an early change from demersal to a semi-pelagic life. P. bovinus will vary its diet by taking cephalopods and teleosts, and among the latter the pelagic species are the most important.
doi: 10.1093/icesjms/37.3.221pmid: N/A
Investigations of the dispersal of the eggs and larvae of the plaice in the Southern Bight of the North Sea have involved measurement of the egg and larval distributions, the dye tracer Rhodamine-B and an array of moored current meters. The spread of the egg and larval distributions changed little as the eggs and larvae developed, suggesting that turbulent diffusion was at a comparatively slow rate. This conclusion was supported by the results of the tracer experiment and the application of a mathematical model of diffusion. In this area the vertical shear diffusion process is much more important than horizontal turbulence in producing dispersal. The current meter results showed that in the central Southern Bight there was little difference between water movements at different geographical positions. Provided that attention was confined to water movements along the main tidal axis, the current meter results and the movement of the Rhodamine patch showed a constant proportionality to water movements through the Dover Strait. A marked difference between the northeasterly drift of larvae in 1962 and 1969 was found to correspond to similar differences in the flow through the Dover Strait.
doi: 10.1093/icesjms/37.3.249pmid: N/A
Various sources of errors in Virtual Population Analysis (VPA) are discussed. Errors in the assumed value of natural mortality create errors in estimated fishing mortalities of about the same size but with opposite sign if the total mortality varies moderately from year to year and the true natural mortality is constant. Stock size will be over- or under-estimated when natural mortility is over- or under-estimated, but estimated relative changes in stock size from year to year will be approximately correct. Fluctuations in total mortality caused by random fluctuations in natural mortality from year to year will disappear almost completely in VPA if a constant natural mortality is assumed. A trend in the true natural mortality with time or age will be converted to a trend in the VPA estimates of fishing mortalities. Relative strength of weak and strong year classes will be wrongly estimated by VPA if the true natural mortality varies with year class strength, and this may influence various regression lines commonly used for predicting year class strength from young fish survey indices. Errors in VPA caused by uneven distribution of natural or fishing mortality throughout the year are shown to be generally small and negligible. Effects of stock migration on VPA are discussed. For a year class which continuously migrates from an area A to an area B at a constant emigration rate, equations which give the number of fish of the year class at any time in each of the two areas are developed. These equations may be utilized in a technique similar to VPA for retrospective analysis, and the application of this technique is illustrated. If emigration is included in the natural mortality the traditional VPA may successfully be used in area A. There is, however, no simple way to adjust for immigration in area B, and the traditional VPA may give a completely wrong picture of the situation in this area.
doi: 10.1093/icesjms/37.3.261pmid: N/A
L'âge de R. batis et de R. naevus a été déterminé par lecture des anneaux de croissance inscrits dans les corps vertebraux. 126 spécimens de R. batis et 217 de R. naevus en provenance de la Mer Celtique ont été examinés. Les observations ont permis la calcul des paramètres de l'équation de croissance:R.batisLt=254[1−exp−0·057(t+1·63)]R.naevusLt=91.64[1−exp−0.108(t+0.46)]Les taux de croissance sont de même ordre que ceux obtenue par Holden (1972) d'expériences de marquage sur trois expèces de raies des eaux britanniques.
doi: 10.1093/icesjms/37.3.266pmid: N/A
A total of 1841 cod and 2543 whiting from Scottish waters were examined for the presence of larval nematodes within the musculature. Larval Phocanema were common in the musculature of cod from the west coast of Scotland but rare in fish from the central northern North Sea. Larval Anisakis were most abundant in the musculature of cod and whiting from offshore northern North Sea areas but were much less common in other areas. Phocanema occurred only very occasionally in whiting. Anisakis and Phocanema rarely occurred together in the musculature of individual cod. Observed variations in infections of fish are possibly due to differences in the geographical distribution of the invertebrate and vertebrate hosts of the parasites. There has apparently been a large increase in the infection of cod and whiting with larval nematodes within the last twenty years probably due mainly to a greater abundance of Anisakis.Infection with larval nematodes increased with age and length of the fish. Most Anisakis from the musculature were found in the hypaxial muscles whereas a much larger proportion of Phocanema occurred in the epaxials. In whiting over half the total burden ofAnisakis were found in the musculature rather than the viscera but in cod most larvae occurred in the latter site. There was a significant positive correlation between numbers of Anisakis in the viscera and musculature of whiting but not of cod. Phocanema was found in a number of fish species which may act as a source of infection for cod.
doi: 10.1093/icesjms/37.3.274pmid: N/A
Fluctuations in the total weight and number of species, and the volume of each of 24 species inthe annual catch of marine and estuarine fisheries of the Maine coast, representing 80% of total production, are highly significantly correlated with sea temperature cycles. Another seven species representing an additional two percent of total annual catch are significantly correlated in relative abundance or availability with the same cycles.
doi: 10.1093/icesjms/37.3.281pmid: N/A
From a large body of data of quanta irradiance (350–700 nm) in different water masses the depth has been derived at which the percentage of surface irradiance of quanta is 30%, 10%, 3%, and 1% respectively. It has been shown that the relationship between the 10% level and the other levels can be expressed by simple equations with an accuracy sufficient for most practical purposes. Thus three of the levels can be computed if one is known. This implies that a classification of sea water in terms of transmittance of quanta irradiance is established for the whole euphotic zone.
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