ICES Journal of Marine Science: Journal du Conseil

Sánchez-Velasco,, L.;Shirasago,, B.

doi: 10.1006/jmsc.1998.0413pmid: N/A

Abstract The purpose of this study is to analyse the spatial distribution of some groups of microzooplankton, including the more common potential prey of fish larvae (e.g. copepod nauplii and copepodites), in relation to oceanographic mesoscale processes in the vicinity of the Palamos Canyon (Catalan Sea, NW Mediterranean) as identified by a set of oceanographic and plankton data (60 μm mesh) obtained during a cruise in May 1992. The area was dominated by the Liguro-Provençal-Catalan current (slope current) that was influenced by the cold, fresh water discharged from the river Rhône and partial perturbations of its flow induced by the Palamos canyon. Spatial distribution of the plankton was examined by cluster analysis. Two main groups of stations and taxa were defined and named the “Oceanic group” and “Neritic group”, respectively. The Oceanic group was characterized by high concentrations of copepod nauplii and copepodites associated with slope current waters and its intrusions into the submarine canyon. In addition the presence of a large cyclonic eddy, detected by satellite data, at the eastern margin of the slope current could contribute to the concentration of organisms in this zone. The Neritic group was located on the shelf and slope areas and characterized by taxa of neritic–meroplanktonic habits together with copepod nauplii and copepodites. The latter were in lower abundances than recorded in the Oceanic group. The results suggest the microzooplankton groups analysed show well-defined distribution patterns related to the slope current and its interaction with the heterogeneity of the shelf. These in turn, affect the distribution of fish larvae and other predators. This content is only available as a PDF. © 1999 International council for the Exploration of the Sea

Helle, K.; Pennington, M.

doi: 10.1006/jmsc.1998.0427pmid: N/A

Arcto-Norwegian cod spawn along the coast off mid and northern Norway in March and April. The eggs and larvae are carried north by the currents and a large proportion of the early juveniles drift into the Barents Sea. Surveys of the early juvenile cod and zooplankton populations were conducted in late June and July from 1978 through 1984 off the coast of northern Norway and in the Barents Sea. Based on this survey series, the spatial distribution of the early juvenile cod was compared with the distribution and abundance of the zooplankton. In all years, there was more zooplankton, on average, in subareas in which early juvenile cod were observed than in those without early juveniles. The average water temperature was higher and salinity was lower in the subareas with cod than in the subareas without cod. The spatial distribution of the early juvenile cod and zooplankton appears to be influenced by the wind-driven flux of water through the section between Fugløya and Bear Island and the correlation between the amount of zooplankton in the Barents Sea and the water flux was positive and significant.

Kappenman, R. F.

doi: 10.1006/jmsc.1998.0422pmid: N/A

A method for estimating mean catch per unit effort (c.p.u.e.) with groundfish trawl survey data is proposed and examined in detail. The new method is a modificaiton of the arithmetic average of the survey c.p.u.e.s. It is meant to be applied to those cases where one or more of the survey c.p.u.e.s are excessively large and exert too much influence on the arithmetic average, when it is used to estimate mean catch per unit effort. These large c.p.u.e.s are replaced by estimates of their expected values.

Hedgepeth, J. B.; Gallucci, V. F.; O'Sullivan, F.; Thorne, R. E.

doi: 10.1006/jmsc.1998.0431pmid: N/A

The smoothed expectation and maximization (EMS) method is shown to be superior to deconvolution, the most commonly used indirect method for estimating fish acoustic size. This is primarily because EMS avoids artifactual modes in estimates of acoustic size. The expectation, maximization and smoothing method starts with the iterative statistical technique called expectation and maximization and adds smoothing between iterations. Application of the EMS method assumes that the acoustic observations in the voltage domain are realizations of a non-homogeneous Poisson process of rate λi. The E-step estimates expected values ofλiusing the conditional distribution of the “complete” data, which is binomial, given the Poisson distributed acoustic observations. The M-step uses the log likelihood function of the Poisson distributed “complete” data to find maximum likelihood estimates of λi. The S-step smoothes theλiestimates, with a Gaussian convolution filter. The method is demonstrated using a simulated data set of known statistical properties and a data set from the acoustics literature.

Bertrand, A.; Josse, E.; Massé, J.

doi: 10.1006/jmsc.1998.0430pmid: N/A

A research programme was carried out in French Polynesia to study tuna behaviour using acoustics and fishing experiments. Acoustics are of great importance for the study of tuna behaviour and estimation of abundance but estimates of individual target strength that are available are particularly inaccurate. In this study, four yellowfin tuna (Thunnus albacares) and two bigeye tuna (Thunnus obesus) of weight 4–50 kg were individually caught, identified, and equipped with ultrasonic tags for telemetry experiments. While tracking the fish, simultaneous underwater acoustic data were recorded with a split-beam echosounder in order to estimate their in situ acoustic target strength. It was observed that target strength was stronger when fish were diving than when they were ascending toward the surface. This can be explained by the tilt angle orientation of the swimbladder. A target strength bias according to depth was also observed.

Kelly, C. J.; Connolly, P. L.; Bracken, J. J.

doi: 10.1006/jmsc.1998.0426pmid: N/A

This paper presents data on the age, growth, maturity, and distribution of Helicolenus d. dactylopterus (Delaroche 1809) from the Rockall Trough area. The results are based on a series of deep-water trawl surveys carried out on the continental slope west of Ireland and Scotland between 1992 and 1993. Ages ranged from 1 to 43 years for males and 1 to 37 years for females with most fish aged between 3–14 years. The species was found to be sexually dimorphic with males attaining a greater length and weight (L∞=37.2 cm; W∞=747 g) than females (L∞=31.0 cm; W∞=700 g). Age and growth estimates were determined for both sexes and the Brody growth coefficient for both sexes indicate that this species is relatively slow growing (male K=0.06; female K=0.09). Ripe female fish were only found in April and May in a specific area. Using total lengths 50% of male fish were mature at 26 cm (ages 15–16) while 50% of female fish were mature at 23 cm (age 13). Catch data were sporadic but the largest catches were obtained on the southern slope of the Rockall Trough in April at a depth of 650–950 m. The fish from this area had a much greater range of sizes than fish from the eastern slopes of the Rockall Trough.

Rasmussen, E. B.; Salhauge, M.-B.; Boje, J.

doi: 10.1006/jmsc.1998.0420pmid: N/A

Vertebral counts of Greenland halibut collected from West Greenland in 1995 were compared to samples from the same areas for the period 1987–1989. The 1995 samples, obtained in Davis Strait, Baffin Bay, and two North-west Greenland fjords, showed no significant differences in mean vertebral numbers. Previous studies in 1987–1989, however, showed significant differences in mean vertebral counts between some of the same areas. Inter-annual variation in vertebral numbers is therefore suggested to be greater than the variation between sample areas. Despite the protracted egg and larval stage of Greenland halibut, the currents along the West Coast of Greenland are unlikely to transport juveniles in any significant amount from the Davis Strait spawning grounds to the North-west Greenland fjords. The origin of the northernmost populations therefore remains unknown. As single cohorts in the material seem to affect the total average counts per area significantly it is suggested that a change in the distribution of Greenland halibut might have occurred during the period studied. The results also emphasize the importance of relying on data based on more than one year and question the suitability of vertebral counts in stock delineation studies of Greenland halibut.

Jensen, A. J.; Zubchenko, A. V.; Heggberget, T. G.; Hvidsten, N. A.; Johnsen, B. O.; Kuzmin, O.; Loenko, A. A.; Lund, R. A.; Martynov, V. G.; Næsje, T. F.; Sharov, A. F.; Økland, F.

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Stergiou, K. I.

doi: 10.1006/jmsc.1998.0421pmid: N/A

In the present study the retained proportion and the mean individual weight was compared for 61 marine demersal species caught with three different trawl codends: the 14 mm (bar length) diamond mesh (14 D) (presently used in the Hellenic commercial trawl fishery), the 20 mm square mesh (20 S), and the 20 mm diamond mesh (20 D). The experiments took place at 12 stations in the western Aegean Sea in October 1993 and March 1994. The results indicated that: (1) 20 D and 20 S allow significantly more individuals to escape through the meshes; and (2) the mean weight of the retained individuals for 20 D is significantly larger than those for 14 D and 20 S. Thus, the banning of 14 D codend from the fishery seems to be essential for the rational management and conservation of the Hellenic demersal resources.

Gaspar, M., B.;Castro,, M.;Monteiro, C., C.

doi: 10.1006/jmsc.1998.0423pmid: N/A

Abstract Experiments on the selectivity of the clam and razor clam dredges were carried out during July 1995, on the south coast of Portugal. The effects of mesh size and tooth spacing were investigated for both the clam and razor clam dredges. Four different mesh sizes (25, 35, 40, and 50 mm) and three different tooth spacings (15, 20, and 25 mm for the clams Spisula solida and Venus striatula, and 10, 15, and 20 mm for the razor clam Ensis siliqua) were used. The results indicated that the tooth spacing did not have an effect on the selectivity. The best mesh sizes were 40 mm for the clam fishery (S. solida and V. striatula) and 66 for the razor clam (E. siliqua). These mesh sizes will result in considerable reduction in the numbers of small- and medium-sized individuals in the catch. This content is only available as a PDF. © 1999 International council for the Exploration of the Sea