ICES Journal of Marine Science: Journal du Conseil

Goddard, G. C.; Welsby, V. G.

doi: 10.1093/icesjms/42.3.197pmid: N/A

Experiments to measure the sonar target strength of free-swimming live fish were made in a sea-flooded quarry, the target fish being constrained within a netting cage which did not affect the tests. The cage was kept at a mean depth of 10 m and the acoustic beam could be trained on it either vertically downwards or at an angle of 22½° to the horizontal. Measurements were made of pulse amplitude, calibrated to an accuracy of ±1 dB, irrespective of the position of the fish in the cage. One thousand successive pulses were recorded in each case and the mean value subsequently computed to give one data point. Species and numbers involved were: cod, Gadus morhua (102); haddock, Melanogrammus aeglefinus (41); saithe, Pollachius virens (8);and dogfish, Squalus acanthius and Scyllium canicula (23). Tests were made at dorsal aspect and at 22½° aspect, at 10, 30, and 100 kHz, respectively, making a total of 6000 individual recorded measurements on each of 174 fish. The results are presented in the form of scatter diagrams and also as empirical regression formulae.An additional experiment investigated the combined target strength of groups of fish containing 2, 4, 8, 16, and 32 individuals, respectively. If each echo were unaffected by the others there should have been a rise of 3 dB per doubling of number of fish. While “shadowing” effects would cause a rise less than 3 dB, the observed results showed an as yet unexplained rise slightly greater than 3 dB per doubling of numbers.

Foote, Kenneth G.

doi: 10.1093/icesjms/42.3.212pmid: N/A

Goddard and Welsby's decade-old study is placed in the context of the contemporary literature. Its experimentation is described as a paradigm of encaged-fish measurement. Two of the authors' findings are emphasized: the necessity of averaging target-strength data in the squared-amplitude or intensity domain, and the importance of observing fish behaviour for understanding fish target strength and its variation. The likely uniqueness and value of the data at 22½° declination are noted. Comparison of the gadoid and dogfish target strengths enhances other work showing that the swimbladder contribution to the fish echo energy generally exceeds 90%. Apparent irregularities in the multiple-fish data are attributed to well-understood behavioural effects that are entirely consistent with the basic linearity of fisheries acoustics.

Mazé, Robert; Camus, Yves; le Tareau, Jean-Yves

doi: 10.1093/icesjms/42.3.221pmid: N/A

L'étude présentée ici a pour objectif de donner une explication de l'existence de la bande d'eau relativement froide qui borde le talus continental du Golfe de Gascogne et qui apparaît, en été, sur les images satellitaires.On se place dans I'hypothèse de I'océan bi-couche, au-dessus d'un talus continental et sur une radiale perpendiculaire à ce talus.Dans une première partie, on simule la formation de la marée interne qui se propage de part et d'autre du talus. Ensuite, quand cette marée interne est bien établie, on simule le brassage mécanique résultant de I'action d'un coup de vent. L'effet du mélange, qui se traduit par un refroidissement de la couche supérieure et par une augmentation de l'amplitude des ondes internes, est plus efficace au-dessus du talus oú l'amplitude de la marée interne est maximale. Il en résulte la formation, dans cette région, d'une bande d'eau relativement froide.L'interaction entre les ondes internes et le mélange apparaît clairement sur des mesures faites au cours de I'opération ENVAT 81. Les résultats obtenus fournissent donc une explication claire de l'existence de cette bande froide durant toute la période pendant laquelle l'océan est stratifié.In recent years infra-red satellite imagery has shown there to be a narrow band of cold water along the Celtic Sea shelf break in summer when the ocean is well stratified. This paper provides an explanation of this phenomenon, which probably occurs more frequently at continental shelf breaks than is usually believed.At first, by means of an analytical model, linear propagation of a monochromatic barotropic tide along a normal to the shelf break in a homogeneous ocean is simulated. Then, in an idealized two-layer ocean, the propagation of the barotropic tide above the continental slope that generates a baroclinic tide at the shelf break is noted. This internal wave propagates to both sides, i.e. above the continental shelf and above the abyssal plain. The energy flux lost by the barotropic tide from the oceanic limit of the continental slope to the shelf-break limit is used by the baroclinic tide for its propagation. By this means, it is possible within a linear theory to compute the internal wave amplitude.The results of this numerical simulation are in good agreement with the time series of temperature observed at three stations (one on the abyssal plain, the second on the shelf break, and the third on the continental shelf) during the Service Hydrographique et Oceanographique de la Marine survey ENVAT 81.Without any heat flux, the turbulent exchange fluxes during a wind impulse are described by the kinetic energy equation. With this condition, a numerical simulation of these turbulent fluxes together with a numerical simulation of the baroclinic tide propagation shows, during the wind impulse:1) a deepening of the mixed layer, the depth of which tends towards a limit depending on various parameters such as wind speed and oceanic stratification,2) a step by step decrease, well correlated with the baroclinic tide, of the mixed layer temperature above the shelf break. This temperature tends towards the same limit — and for the same reasons — as the depth of the mixed layer.Consequently, a horizontal temperature gradient appears on both sides of the shelf break.These numerical results are again confirmed by the time series of temperature observed during the ENVAT 81 survey.

Smith, R. L.; Paul, A. J.; Paul, J. M.

doi: 10.1093/icesjms/42.3.241pmid: N/A

This paper summarizes an investigation of food intake and its effect on growth, conversion efficiency, and condition factor of juvenile walleye pollock (Theragra chalcogramma) maintained in captivity in Seward, Alaska. Observations were made at 3 to 7–5°C, the range pollock inhabit in the Bering Sea for most of the year. Food consumption was measured as weight eaten in percentage of body weight per day and also in calories per day.Maximum food consumption (% b.w./day) and maximum growth (weight gain as a % b.w./day) were both negative exponential functions of pollock weight and inverse linear functions of pollock length. Maximum consumption in calories per day was a negative exponential function of pollock weight.In 30–60 g pollock, growth was a linear function of food consumed whether consumption is expressed as % b.w./day or as calories per day. Maximum growth at 3 and 7·5°C was 0·93 % and 2·12 % of body weight per day, respectively. Maintenance ration in this size range was 225 calories per day or 0·26 % b.w./day at 3°C and 382 calories per day or 0·30% b.w./day at 7·5°C. Change in condition factor was a direct, linear function of food intake (r2 = 0·97 at 3°C; r2 = 0·87 at 7·5°C), making it possible to calculate the ration required to maintain a constant condition factor. Those required rations are three to four times higher than maintenance rations, suggesting that juveniles, in contrast to adults, preferentially grow in length at low levels of food consumption.During starvation, pollock lose weight at a constant rate (—0·34% of body weight per day at 8°C) and, therefore, condition factor declines linearly with the duration of starvation. Death of juveniles due to starvation occurs at a condition factor of about 0·44.Between 3 and 7·5°C, both maximum food intake and growth were linearly related to temperature. Maximum conversion efficiency was an inverse, linear function of temperature (90 % at 3°C; 37 % at 7·5°C), suggesting that at low levels of food availability, growth is more rapid at colder temperatures. These last results are discussed in relation to the movement by juvenile pollock into colder water during the spring in the Bering Sea.

Gee, J. M.; Davey, J. T.

doi: 10.1093/icesjms/42.3.254pmid: N/A

The stages of development of Mytilicola intestinalis consist of a nauplius, metanauplius, copepodites I–V, and a preadult before the sexually mature adult. The rates of development of the embryonic, free-living, and parasitic phases of the life cycle have been quantified in relation to temperature in a series of laboratory experiments. These show that development times, particularly of the embryos and parasitic instars. are increased significantly at temperatures below 18°C but not significantly decreased at temperatures above 18°C. It is suggested that the influence of temperature on rates of development is the mechanism by which the seasonal oscillations in the population of M. intestinalis are controlled rather than invoking the idea of a discrete temperature threshold for the onset or cessation of breeding as had been postulated previously.

Gee, J. M.; Davey, J. T.

doi: 10.1093/icesjms/42.3.265pmid: N/A

Infestation is the most critical phase in the life cycle of Mylilicola intestinalis and is shown to be essentially a passive process dependent first on the chance encounter between larvae and the host's field of filtration and secondly on the strength of the host's inhalant current. An examination is made of the effects of temperature, the age and availability of infestive larvae, and the size, density, and previous infestation history of the hosts, on infestation success. It is concluded that host density and the size of the population of infestive larvae are most important in determining immigration into the adult population and that the size of the parasite population in individual hosts may be density-dependent. Finally the essentially commensal characteristics of the relationship between M. intestinalis and its host are discussed.

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

doi: 10.1093/icesjms/42.3.272pmid: N/A

A model is developed which describes recruitment in haddock by the separate effects of total stock and spawning stock. Evidence is produced which indicates that whilst spawning stock makes an approximately proportionate contribution to egg production, the total stock has an adverse effect on subsequent recruitment. Pre-recruit mortality, which is related to total stock size, appears to be the operative factor. The model can account for about 40% of the variation in log recruitment. In whiting about 22% of the variation in log recruitment can be attributed to the negative effect of total stock size. There is no detectable separate spawning-stock effect.

Corten, A.

doi: 10.1093/icesjms/42.3.281pmid: N/A

An analysis of stock and recruitment data for individual North Sea herring stocks shows that the recruitment failure in the 1970s cannot be completely ascribed to a low spawning-stock size. Other explanations, such as selective removal of juvenile herring by the industrial fishery for sprat, or increased competition from other species, are also shown to be incompatible with various data presently available. A new hypothesis presented here ascribes the recruitment failure to a disruption of the transport of herring larvae, caused by a change in North Sea circulation. Results from an international Isaacs-Kidd Midwater Trawl sampling programme, together with data from a sampling programme for herring larvae in Dutch coastal waters, are presented to support this hypothesis.

Andersen, K. P.

doi: 10.1093/icesjms/42.3.295pmid: N/A

Bucke, D.

doi: 10.1093/icesjms/42.3.297-apmid: N/A