The first spontaneous spawning of European hake Merluccius merluccius L.: characteristics of eggs and early larval stages

For the first time, a spontaneous spawning of hake was recorded in Spain in April 2009. The spawn was obtained from broodstock kept in captivity for two years at the facilities of the Spanish Institute of Oceanography in Vigo (NW Spain). Eggs were transparent, spherical and had an average diameter of 1.067 ± 0.024 mm; yolk occupied the majority of egg volume. The oil droplet had a diameter of 0.27 ± 0.03 mm. The incubation period of the eggs lasted for 4 days at 14°C and the duration from hatching to the total absorption of the yolk sac was between 5–7 days after hatching, at the same temperature. Newly hatched larvae had an average total length of 3.20 ± 0.13 mm and began feeding 6 days after hatching; a daily growth rate of 0.158 mm day‐1 was observed from hatching to yolk sac consumption. This paper describes the daily evolution of biometric and morphological characteristics of the different stages of embryos and larvae of European hake up to the age of 19 days.     

The effect of temperature on the development of yolk‐sac larvae of European hake (Merluccius merluccius L.) under laboratory conditions

The effect of temperature (10, 13, 16, 19 and 22°C) on hatching, development and survival of yolk‐sac larvae of European hake, Merluccius merluccius, was studied. At 22°C the experiment was suspended because all eggs died a few hours after incubation. Five morphological indicators of larval development (standard length – SL, yolk‐sac volume – YSV, oil globule diameter – OD, body height – BH and body wet weight – BW) were analysed. SL, YSV, OD and BW of newly hatched larvae were biggest in size and weight at low temperatures (P < 0.05). Throughout the experiments, the rates of changes in SL, YSV and OD varied with temperature (P < 0.05). By contrast, BH and BW remained constant and did not show significant relationship with the effect of temperature (P > 0.05). The potential model showed that the SL growth rate varied from 0.05 to 0.08 mm day^?1 from 10.5 to 19.5°C respectively. However, at all temperatures two growth phases were identified: a rapid growth phase followed by a slow one. Rapid growth rate in length and depletion in yolk‐sac and oil globule occurred at high temperatures. After total YSV and OD consumption (death by starvation), larvae died between 8 and 14 days at 19.5 and 10.5°C respectively, reaching a maximum length of 4.3 mm at 10.5°C. Metabolic indices such as: lowest threshold temperature (T₀); effective larval development (); cumulative thermal units (TC) to complete larval development; the temperature coefficient on growth rate in length (Q₁₀) and the condition factor (K) were also studied.     

European hake fishery bioeconomic management (southern stock) applying an effort tax

A bioeconomic model is developed to illustrate the way in which a tax on effort applied to a multi-fleet fishery can obtain socially optimum operating results. The analysis is applied to the European southern hake stock. We demonstrate how including heterogeneous fishermen, working simultaneously in the fishery, will lead to different equilibrium levels for the tax to be applied to each of them. Using information from the fishery which is the object of the study, the most relevant results and estimations of hake biomass, catches and effort are shown. Lastly, we make some recommendations on economic policy for this fishery.     

Links between the recruitment success of northern European hake (Merluccius merluccius L.) and a regime shift on the NE Atlantic continental shelf

The distribution of northern European hake (Merluccius merluccius L.) extends from the Bay of Biscay up to Norwegian waters. However, despite its wide geographical distribution, there have been few studies on fluctuations in the European hake populations. Marine ecosystem shifts have been investigated worldwide and their influence on trophic levels has been studied, from top predator fish populations down to planktonic prey species, but there is little information on the effect of atmosphere–ocean shifts on European hake. This work analyses hake recruitment success (recruits per adult biomass) in relation to environmental changes over the period 1978–2006 in order to determine whether the regime shift identified in several abiotic and biotic variables in the North Sea also affected the Northeast Atlantic shelf oceanography. Hake recruitment success as well as parameters such as the sea surface temperature, wind patterns and copepod abundance changed significantly at the end of the 1980s, demonstrating an ecological regime shift in the Northeast Atlantic. Despite the low reproductive biomass recorded during the last decades, hake recruitment success has been higher since the change in 1989/90. The higher productivity may have sustained the population despite the intense fishing pressure; copepod abundance, warmer water temperatures and moderate eastward transport were found to be beneficial. In conclusion, in 1988/89 the Northeast Atlantic environment shifted to a favourable regime for northern hake production. This study supports the hypothesis that the hydro‐climatic regime shift that affected the North Sea in the late 1980s may have influenced a wider region, such as the Northeast Atlantic.     

Patchiness of eggs, larvae and juveniles of European hake Merluccius merluccius from the NW Mediterranean

We estimated patchiness for different life stages (egg, larvae and juveniles) of European hake (Merluccius merluccius) from two ichthyoplankton surveys (September and November 1999) and four bottom‐trawl surveys (November 1998, February, June and September 1999). Lloyd's patchiness P = 1 + 1/k, where k is the dispersion parameter of the negative binomial distribution, was calculated. Juveniles showed lower indices of patchiness than early ontogenetic stages. In juveniles, patchiness was highest for smaller size classes (3.5–5 cm TL) settling to their near‐bottom habitat and was very low for those larger than 5 cm up to the 30–35 cm size class (approximately 3 yr of age, 50% mature individuals). Patchiness of settling juveniles was especially high in the June and September surveys, coinciding with peak recruitment to the bottom. Both eggs and yolk sac larvae showed patchiness values <10, increasing in flexion stage larvae from 12 to 20. This pattern is different to that observed in other studies for pelagic species, where patchiness of early‐stage eggs is very high, but coincides with results obtained for other demersal species with pelagic eggs. Factors that may explain these patterns are discussed in light of oceanographic conditions, adult distribution and larval ontogenesis.     

Spatial and temporal distribution and abundance of European hake, Merluccius merluccius, eggs and larvae in the Catalan coast (NW Mediterranean)

Plankton hauls were conducted on five surveys from November 1998 to November 1999 to study the spatial distribution of eggs and larvae of European hake off Catalonia (NW Mediterranean Sea). Also CTD casts to record hydrographic parameters were carried out on a closely spaced station grid. Merluccius merluccius eggs and larvae appeared mainly in late spring, summer and autumn surveys and were very scarce in winter. Strong differences in terms of egg and larval densities were observed between the two November surveys, which could be attributed to the anomalous hydrographic situation during November 1998. M. merluccius egg and larvae were mainly distributed over the continental shelf, with peak abundances between the 100 m isobath and the edge of the shelf. On the evidence of larval size frequency distributions in the different sampling sectors and the closely overlapping distribution patterns for the eggs and the adult spawning stock, drifting of hake eggs and larvae was not a major factor. The larval distribution extended only slightly further offshore than the egg distribution. Using the hydrographic information and the larval distribution data, an attempt was made to relate the different seasonal productivity levels over the spawning period and the distribution of the larvae.     

Age determination and growth of juveniles of the European hake,Merluccius merluccius (L., 1758), in the northern Tyrrhenian Sea (NW Mediterranean)

The aim of the present study was to provide an estimation of growth of juvenile European hake, Merluccius merluccius (L., 1758) (OSTEICHTHYES; MERLUCCIIDAE), by means of the analysis of otolith daily increments. Hake specimens were collected during trawl surveys carried out in the northern Tyrrhenian Sea (NW Mediterranean). The sagittae were removed from hakes ≤20 cm total length. Left otoliths were ground and polished to obtain thin frontal sections. Otolith microstructure was analysed under a compound green light-polarising microscope. A power curve with intercept was fitted to the length-age data to describe the growth of M. merluccius. According to the growth curve, a mean length of 18 cm was reached at the end of the first year of life. The validation of the otolith increment periodicity was performed by means of two indirect methods.     

Larval development and fatty acid composition of Ostrea edulis (L.) fed four different single diets from conditioning to pre‐settlement

Survival, growth and fatty acid composition of Ostrea edulis larvae (L.) fed four different single species, microalgal diets, Tisochrysis lutea (T), Chaetoceros neogracile (Cg), Skeletonema marinoi (Sm) or Tetraselmis suecica (Ts) from broodstock to pre‐settlement, were studied. Lower larval growth (5.5 μm to 6.5 μm/d) was recorded in progeny continuously fed single S. marinoi or T. suecica, whereas good growth was achieved with single T. lutea (7.8 μm/d). Larvae, originated from broodstock receiving Sm or Ts, exhibited growth compensation when fed a bispecific balanced diet (TCg). This did not occur when broodstock and larvae were fed Cg or T, for which single or mixed diets led to similar larval growth. Furthermore, survival was high (>90%) regardless of microalgal diet, except for larvae fed from broodstock to pre‐settlement T (53%) or Ts (2%). There were significant differences in 20:5 (n‐3) and 22: 6 (n‐3) contents in polar and neutral fractions of O. edulis expelled larvae dependent on broodstock diet, as well as throughout larval development, but no clear trend was apparent when comparing fatty acid (FA) relative composition of both fractions of O. edulis larvae fed different diets at release or prior to settlement. In contrast, such correlation occurred when FA was expressed in absolute content but exclusively for larvae‐fed single diets and was particularly noticeable between 22: 6 (n‐3) and growth and survival. In the present work, broodstock nutritional deficiencies have been revealed in O. edulis progeny, compensated thereafter by feeding the larvae a mixed diet, and in this balanced condition, no obvious relation with larval development indicators was found with main fatty acid contents.     

Digestive enzyme activity of the red porgy (Pagrus pagrus, L.) during larval development under culture conditions

The ontogenic development of the main digestive enzymes (proteases, amylase and lipase) in the red porgy, Pagrus pagrus, larvae was assayed during the larval development. The green water technique was carried out for larval rearing and whole‐body homogenates were used for enzymatic assays in triplicate. Significant alterations in specific activities of all digestive enzymes measured during the period of this study were mostly related to metamorphosis and weaning. Trypsin‐ and chymotrypsin‐specific activities were first detected on day 3, together with opening of the mouth, and slightly increased until 25 days after hatching (DAH). After this period, the specific activities of these enzymes slightly decreased. Pepsin was first detected on day 28, concurrent with stomach formation, and a sharp increase was observed until 30 DAH. A slight decrease was measured from this date until the end of the experiment. Both amylase and lipase were measured for the first time on days 2 and 4 respectively, and the specific activities of these enzymes showed similar patterns during the first week of the study. Then, slight variations were observed until 30 DAH and while lipase‐specific activity declined, an increase in the specific activity of amylase was found until the end of the experiment. It is concluded that the variations observed in the specific activity of digestive enzymes were related to either metamorphosis, such as the formation of the stomach (28 DAH), or to changes in food composition. The profile of the developmental pattern of the main digestive enzymes detected in P. pagrus is similar to that described for other Sparid species.     

Development of digestive system and swim bladder of larval nase (Chondrostoma nasus L.)

Digestive tract and swim bladder differentiation during nase (Chondrostoma nasus L.) larval development was evaluated using histological methods. The mouth, pharynx and oesophagus were impervious at the time of hatching. The straight tubular intestine consisted of undifferentiated cells and was situated over the yolk sac. The intestine, lined with a single layer of columnar epithelial cells, opened on 2 day posthatching (dph). First evidence of protein digestion and absorption by the enterocytes was observed on the sixth day, while lipid digestion and absorption began on the ninth day of larval development. Intestinal mucous cells developed and became active by the fourth day. The posterior chamber of the swim bladder inflated on the third day, while the anterior one filled 12 dph. Complete yolk sac resorption took place on the ninth day.     

Post-stocking dispersal of European grayling,Thymallus thymallus (L.), in a semi-natural experimental stream

Abstract Artificially hatched and reared juvenile European grayling, Thymallus thymallus (L.), from three natural populations were released into an experimental stream. Two of the populations were from lakes and the third population was from a river. Post-stocking dispersal was studied by recapturing fish in traps and by electric fishing after each of the stocking exercises. On all three occasions the first fish to be recaptured were caught in a trap about 400 m downstream from the release site, within 5–12 min of release. Proportions of stocked grayling juveniles recaptured by electric fishing in the fast-flowing section of the experimental stream (the rapids) were 9.9%, 46.7% and 16.6% after the first, second and third stocking, respectively. No significant population-specific difference was found between fish caught in the downstream trap and fish recaptured in the rapids. The uppermost part of the rapids (stretch 1) and the catch in the upstream trap was dominated by fish of riverine origin. Post-stocking migration was influenced by sex but not by body size. Females showed a greater propensity to stay in the rapids compared with males. Implications of the results for stocking grayling in natural streams and regulated rivers are discussed.     

Feed discrimination and selection in self-fed European sea bass Dicentrarchus labrax L.

Abstract This study was conducted to evaluate feed discrimination and preference of European sea bass Dicentrarchus labrax L., taking into account the effects of feed location, previous feeding experience and light conditions. Fish (63 g) were held in tanks and exposed to a 12L:12D photoperiod and ambient temperature. The following feeding practices were applied in triplicate: three‐choice feeding (self‐ feeding with feeds formulated for carnivorous, herbivorous and omnivorous fish rotated on a weekly basis) and monofeeding (self‐feeding with one of these feeds). After 9 weeks (phase I), fish previously held in monofeeding, were subjected to three‐choice feeding for 2 weeks (phase II). In phase I, fish discriminated between feeds after their rotational displacements and preferred the feed formulated for carnivorous species. This also supported the best growth. Fish used in phase II preferred the same feed. There was no preference for any location within tanks. Feeding was generally diurnal, although in some tests with the feeds for herbivores and omnivores, diurnal and nocturnal feed demands were similar and fewer in number than with the feed formulated for carnivores.     

Movement behaviour of large female yellow European eel (Anguilla anguilla L.) in a freshwater polder area

Due to a recruitment decline of more than 90% in 30 years, the European eel (Anguilla anguilla L.) has been classified by IUCN as critically endangered. Although the species has been studied intensively to obtain knowledge to improve management, studies about the resident yellow stage are relatively scarce. In this study, 52 large female yellow eels were tagged with acoustic transmitters in a Belgian polder system and tracked by a network of 23 automatic listening stations. We studied both circadian and seasonal movement patterns and the effect of environmental variables on these patterns. Large female yellow eels were most active at night in late summer and early autumn. A generalised linear mixed model showed that their movement is only slightly influenced by environmental variables. Moreover, as yellow eels show high site fidelity (i.e., the majority was detected only in the habitat type of their catch‐release location), they do not encounter many human‐induced connectivity problems in polder systems, which makes these systems highly suitable as eel growth habitat. These results can contribute to an effective eel management regarding habitat protection and restoration.     

Seasonal changes in selected muscle quality parameters in Atlantic salmon (Salmo salar L.) reared under natural and continuous light

In order to investigate how seasonal variation in growth affects selected fillet quality parameters, immature Atlantic salmon (Salmo salar L.) were reared under simulated natural photoperiod (SNP) for 12 months or continuous light (LL) from January to June followed by SNP until December. Photoperiod treatments advanced the growth rate pattern of the LL group compared with the SNP group and influenced macronutrient metabolism, evaluated both as trends in protein and lipid retention and in fillet lipid and protein levels. Good growth was associated with low fillet lipid and protein level, in addition to reduced levels of fillet tocopherol and astaxanthin, indicating increased oxidative stress. Elevated levels of thiobarbituric reactive substances (TBARs) further supported this. Slaughtering during periods of high growth may therefore reduce postmortem quality, both because of increased susceptibility to fillet lipid peroxidation and reduced astaxanthin levels, which were lowered in vivo and might consequently be depleted further after slaughter. Specialized use of antioxidant‐rich feed prior to slaughter is suggested if slaughtering is expected to occur during periods of high growth rate.     

Simulating the dispersion of vertically migrating sprat larvae (Sprattus sprattus (L.)) in the German Bight with a circulation and transport model system

A model system consisting of two three-dimensional circulation models and a three-dimensional transport model is used to simulate the dispersal of sprat larvae (Sprattus sprattus (L.)) in the German Bight. A Northwest European shelf sea model simulates the currents of the North Sea and provides sea surface elevation data for the open boundaries of the German Bight circulation model. The German Bight circulation model has a horizontal grid resolution of 2.75 km and a vertical resolution of 5 m except for the bottom layer. The driving forces of the circulation models are the M2-tide, six-hourly time-dependent wind stress and air pressure fields as well as monthly climatological density fields. The simulated currents serve as input to the transport model. The model area, grid size and the vertical resolution of the transport model are in accordance with the German Bight circulation model. The transport model simulates advection and diffusion using a Monte Carlo method. Furthermore it incorporates a simulation of active vertical movement of the larvae. The vertical migration pattern is dependent on the time of day and size of the larvae and is based on field data from the German Bight. Horizontal larvae distribution data, gained from two out of five field surveys carried out in the German Bight during spring and summer 1991, are used as initial values for the starting points of the tracers in the transport model. Simulated tracer distributions are compared with larval distributions observed during subsequent cruises about 3 weeks later. Discrepancies and similarities between tracer and larval distributions are discussed.     

Differences in head shape of the European eel, Anguilla anguilla (L.)

Differences in eel, Anguilla anguilla (L.), head shape were the result of greater increases in head width, rather than changes in head length. The ratio of head width:total length (HW:TL) increased significantly ( t -test, P < 0.05) from the glass-yellow eel stage. Cultured yellow eels were exclusively narrow headed (mean HW:TL = 0.027, range = 0.023–0.032), while wild yellow eels displayed an array of head shapes (mean HW:TL = 0.033, range = 0.023–0.046). Therefore, broad heads (HW:TL 0.033) occurred only among wild yellow eels sampled and may have resulted from diet. Cultured yellow eels consumed only small pellet material. Of wild yellow eels stomachs containing food, 78% of broad-headed eels consumed large and/or hard-bodied organisms (e.g. beetles, fish, molluscs and Notonecta sp.), while 83% of narrow-headed eels consumed exclusively small/soft-bodied prey (e.g. amphipods and chironomids).     

Stimulation of gonadal and germ cell development in larval and juvenile carp (Cyprinus carpio L.) by homologous pituitary extract

The data presented show that in larval carp, gonadal size has increased distinctly after treatment with homologous pituitary extract (PE). Moreover, the precocious onset of primordial germ cell proliferation and of sex differentiation into male and female gonads was induced. Body weight of treated and control specimens did not differ significantly from each other throughout the experiment. Treatment of juvenile carp with homologous PE led also to an increase in gonadal size without concomitant increase in body weight. Induction of precocious spermatogenesis was observed too. GTH-levels in control and PE-treated carp were monitored by means of a homologous radioimmunoassay, showing that in PE-treated carp the GTH-level was distinctly elevated. The possible role of pituitary hormones in larval and juvenile gonadal development is discussed.