Sexual maturity in the velvet swimming crab Necora puber (Brachyura, Portunidae): morphometric and reproductive analysesGonzález-Gurriarán, Eduardo; Freire, Juan
doi: 10.1006/jmsc.1994.1015pmid: N/A
Relative growth and morphometric maturity of males and females of Necora puber in soft-bottom mussel raft areas of the Rıa de Arousa (Galicia, north-west Spain) were analysed. A study was made of the changes in the allometric growth of cheliped size, abdominal width of females, and length of the first pleopod in males, related to the onset of maturity. Multivariate analysis and different bivariate regression methods were employed to compare one- and two-phase growth models. Heterochely and allometric changes related to sexual maturity were the main factors determining the variability of the relative growth in both sexes. In males, the length of the first pleopod and the length of the first chela showed a change in allometry. In females, the abdominal width was characterized by two phases of growth. Morphometric maturity size was similar for both sexes (53.3-mm carapace width in males and 52.3-mm in females). Complementary information from the same population on sexual maturity (based on percentage of ovigerous females, presence of sperm plugs, and sperm in the spermathecae and gonad development) indicated a maturity size of 54.8-mm carapace width in males and 49.8-mm in females. The size at onset of maturity based in morphometric and reproductive criteria coincided with a decrease in growth rate at moult in females (transition point 50.5-mm carapace width).
Lipid and elemental composition of sprat (Sprattus sprattus) larvae at mixed and stratified sites in the German Bight of the North SeaHåkanson, J. L.; Coombs, S. H.; Ré, P.
doi: 10.1006/jmsc.1994.1016pmid: N/A
Larvae of sprat (Sprattus sprattus) from mixed and stratified water column sites in the German Bight of the North Sea were assessed for nutritional condition by analysis of total carbon and nitrogen and the lipid fractions triacylglycerol (TAG), cholesterol, and polar lipids. There was no indication of any significant variation in condition index related to diel sampling periodicity, based either on percentage carbon composition or TAG/cholesterol ratio. Inclusion of growth data from otolith ring counts with the carbon and lipid results identified larvae at 9-10 mm in length as being in relatively poor condition. There was some evidence from carbon analysis to suggest that larvae in poor condition were more prevalent at the stratified site than the mixed; however, microzooplankton analysis showed mean food abundance for larvae was slightly higher at the mixed site. The results from the condition analyses and the validity of the methods is discussed in relation to studies of survival of fish larvae.
Composition and fate of the catch and bycatch in the Farne Deep (North Sea) Nephrops fisheryEvans, S. M.; Hunter, J. E.; Elizal, ; Wahju, R. I.
doi: 10.1006/jmsc.1994.1017pmid: N/A
Landings of Nephrops in the North Sea Farne Deep Nephrops fishery represented only 12% of the original catch by weight. The bycatch included 34 species of fish and 23 invertebrate taxa, as well as undersized and unmarketable Nephrops. There were small landings of lemon sole, plaice, whiting, cod, haddock, and starry ray, but the large majority of the bycatch was of juvenile fish or species of no commercial value and they were discarded. Survival of discarded Nephrops and fish was probably minimal. The discards were usually on deck for several hours before they were thrown overboard, by which time the large majority of fish and a high proportion of Nephrops were dead. Nephrops and some fish sank when they were returned to seawater, but other species of fish floated on the surface. In all cases, seabirds took the majority of discards (<70%) when they were thrown overboard. Those Nephrops landing on the seafloor alive are likely to have suffered further mortality because: (1) the bycatch was often discarded distant from the fishing grounds (i.e. the natural habitat), at places where conditions were probably unsuitable for their survival; (2) a high proportion of Nephrops was injured in the trawl and it is evident from laboratory tests that such injuries may cause death several day later; and (3) laboratory studies suggest that injured Nephrops may have difficulty re-establishing themselves on the seafloor because they compete unsuccessfully with conspecifics for food or shelter. There is concern that, in addition to the impact of the Nephrops fishery on stocks of Nephrops and the commercially important fish (especially whiting) which are caught in large numbers in the bycatch, it may have profound effects on the ecology of the seabed. The fishery effectively transfers organic matter from the seafloor to the surface of the sea, where most of it is removed for human food or by seabirds. Little of the fish or Nephrops catch returns to the seabed, dead or alive.
An hypothesis concerning the space-time succession of Calanus finmarchicus in the northern North SeaBackhaus, Jan O.; Harms, Ingo H.; Krause, Michael; Heath, Michael R.
doi: 10.1006/jmsc.1994.1018pmid: N/A
An hypothesis which explains the space-time succession of Calanus finmarchicus in northern European waters is presented. The hypothesis is based on observations of the regional abundance and behaviour of C. finmarchicus during its life cycle, and on knowledge of the water mass distribution and general circulation of the North Sea and of the adjacent continental slope margins. The basis for the hypothesis is supported by modelling of the general features of the continental slope dynamics. An integral part of the hypothesis is the existence of a poleward-flowing eastern boundary jet at the north-west European shelf edge and a counterflow at depth. The final conclusion is that the succession of C. finmarchicus can be explained by a series of space-time cycles along the continental slope of the North Sea and the southern Norwegian Sea in which an intricate combination of hydrodynamics and the migration behaviour of this species play an important role.
Rapport ARN/ADN et évaluation de l'état nutritionnel et de la croissance des larves de poissons marins: un essai de mise au point expérimentale chez la sole (Solea solea L.)Bergeron, Jean-Pierre; Boulhic, Michel
doi: 10.1006/jmsc.1994.1019pmid: N/A
Much work has been and is still devoted to applications of nuclei acid measurements, and especially the RNA/DNA ratio, in marine biology, but several papers show a variability that makes this ration probably unsuitable to some field applications. Five rearing experiments were carried out with Dover sole (Solea solea L.) larvae, three of them in the same standard conditions at 19°C and two others at temperatures closer to those found under field conditions (12 and 13°C). The temperature effect on growth is in accordance with general patterns. However, during ontogenetic development, the RNA/DNA ratio exhibits a very strong variability which is not reproducible in different experiments performed under the same conditions. The main consequence is that no relationship is found between this ratio and observed instantaneous growth rates. In starved larvae, the RNA/DNA ratio rapidly reaches low values, which are dependent on temperature and age, but in the same range as in fed larvae during the first days after hatching. These results are discussed in relation to the literature that advocates the use of this ratio as an index of nutritional status or growth of fish larvae. It is suggested that great care should be taken in using the RNA/DNA ratio, which cannot be considered as a reliable index of growth.
In search of thresholds for recruitment overfishingMyers, R. A.; Rosenberg, A. A.; Mace, P. M.; Barrowman, N.; Restrepo, V. R.
doi: 10.1006/jmsc.1994.1020pmid: N/A
In this study we consider the problem of estimating, for management purposes, a minimum biomass reference level at which recruitment to a fish stock is seriously reduced. We take an empirical, comparative approach to the problem by examining observations on a wide range of fish stocks. Eight methods for estimating spawning stock biomass thresholds for recruitment overfishing are investigated. Their behaviour is tested using stock and recruitment data for 72 finfish populations, each with at least 20 years of data. We considered three classes of thresholds defined by: (1) the stock size corresponding to 50% of the maximum predicted average recruitment; (2) the minimum stock size that would produce a good year class when environmental conditions are favourable; and (3) the stock size corresponding to 20% of various estimates of virgin stock size. The estimators of the first type are generally preferable because they are easily understood, relatively robust if only data at low stock sizes are available, and almost always result in higher levels of recruitment above the threshold.
Movements of cod (Gadus morhua L.) in relation to the tidal streams in the southern North SeaArnold, G. P.; Walker, M. Greer; Emerson, L. S.; Holford, B. H.
doi: 10.1006/jmsc.1994.1021pmid: N/A
Twenty-four cod fitted with 300 kHz transponding acoustic tags were tracked by sector-scanning sonar in the southern North Sea for periods up to 52 h and over distances up to 72 km. Three fish were released at the surface; the others were released on the sea bed at depths of 24 to 73 m after a period of enforced pressure adaptation in a small cage. Tidal currents, which were measured with moored current meters during five tracks, strongly influenced all movements. The three fish released at the surface were transported to and fro over the ground by successive tides, as were five fish that moved net distances of 15 to 20 km off-shore across the tidal stream axis when released close to the East Anglian coast. Three cod released east of the Norfolk Banks moved net distances of 40 to 70 km to the north by selective tidal stream transport. Another released north of the Banks swam to the east along the tidal stream axis. Most of these cod turned to head against the prevailing tide when they went to the seabed and one that showed tidal stream transport in an area of moderate tidal currents made ground against the opposing tide when it was on the bottom. Several fish maintained a heading in midwater for a number of hours and one deviated by no more than ± 45° from its mean heading during an 8-h period. Three fish made slow sweeping turns in midwater to adopt a downtide heading after previously swimming across the tide. Average swimming speeds in midwater were 0.3-0.9 L s−1 and similar speeds were estimated for fish in the bottom boundary layer; ground speeds, which reflected the speed of the tidal current, were proportionately greater than through-water speeds. Several fish made pronounced vertical movements to or from the seabed at or near sunrise or sunset and one fish showed a diel pattern of vertical migration. The results are discussed in relation to the sensory cues and clues that might be used for orientation and migration, swimming performance, and the role of selective tidal stream transport in the spawning migrations of cod in the southern North Sea.