Transport of larvae and juvenile fish into central and northern Norwegian waters

The distribution of juvenile fish in Norwegian waters in spring and summer provides evidence that some of these are transported into Norwegian coastal waters from spawning grounds outside the ecosystems which comprise the central and northern Norwegian coast, the Barents Sea and the Spitsbergen area. The relevant species are capelin (Mallotus villosus Müller), haddock (Melanogrammus aeglefinus L.), herring (Clupea hareri' gus L.) and saithe (Pollachius virens L.). A substantial part of the saithe and haddock stock along the central and northern Norwegian coast is considered to be recruited from spawning grounds in the northern North Sea. A contribution of these species from the spawning grounds around the Faeroe Islands and west of Scotland may also occur in some years. Herring from the northern North Sea are found scattered off the Norwegian coast north to 70^oN. It seems likely that pelagic species such as capelin and herring are transported by the prevailing currents from Iceland.     

Insecticidal activity of the pyrimidine nucleoside analogue (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU)

The insecticidal activity of the antiherpetic agent (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU) was assessed in in vivo assays against the fall armyworm, Spodoptera frugiperda (JE Smith) (Lepidoptera, Noctuidae). BVDU, mixed into an artificial diet, caused a variety of effects, depending on the concentration used. Compared with controls, food intake was lower, larval growth was retarded and larval development was prolonged. The treated larvae formed smaller pupae and the hatching moths often showed morphogenetic defects. A higher mortality could be found in larval and pupal stages and was generally caused by moult disruption. A choice assay showed that BVDU has very slight feeding-deterrent properties, which only partly explain the toxic effects. The agent most probably acts through its cytostatic activity that has been described previously using cell lines of different insect species.     

Effects of methoxyfenozide on Lobesia botrana Den & Schiff (Lepidoptera: Tortricidae) egg, larval and adult stages

The effect of the non-steroidal ecdysone agonist methoxyfenozide was evaluated against different developmental stages of the grape berry moth, Lobesia botrana Dennis & Schiffermuller (Lep, Tortricidae). Methoxyfenozide administered orally reduced the fecundity and fertility of adults treated with 1, 5 and 10 mg litre(-1); longevity was not affected. An LC(50) value of 4.5 mg litre(-1) was obtained when applied to eggs of less than 1 day old. Surface treatment was more effective than when applied by spraying. Administered into the diet, methoxyfenozide had a larvicidal effect; older larvae were more susceptible than younger larvae, with LC(50) values of 0.1 mg litre(-1) for L(1), 0.04 for L(3) and 0.02 for L(5). Larvae treated with sub-lethal doses throughout their lives did not emerge as adults at the highest doses (0.08, 0.04, 0.02 and 0.01 mg litre(-1)), with 65% and 40% emergence occurring for the lowest (0.005 and 0.0025 mg litre(-1)). Mortality occurred only in the larval stage.     

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.     

Anthelmintic Resistance on an Organized Sheep Farm in India

Monitoring anthelmintic resistance in strongyle nematodes by the faecal egg count reduction test and a commercial larval development assay on an organized sheep farm in the semi-arid area of Rajasthan revealed the emergence of resistance to benzimidazoles and rafoxanide and a potential risk of the development of levamisole/tetramisole resistance. A benzimidazole/levamisole combination, avermectins and closantel were each found to be efficacious.     

Effects of larval size and hydrodynamics on the growth rates of the black fly Simulium tribulatum

Black flies are ubiquitous and important members of lotic ecosystems. Size is known to affect many aspects of their life in the aquatic larval stage, including intraspecific competition for feeding sites. As filter feeders, flow affects their ability to feed and reach sufficiently fast flow. This, in turn, can lead to risky fluid‐mediated dispersal behavior in search of better conditions. It is surprising, therefore, that little information is available regarding how physiological and environmental factors combine to affect larval growth rates. The present study determines the relative growth rates of small (0.6 mm) and large (approximately 4 mm) larvae in laboratory flumes designed to produce spatially homogeneous and temporally consistent flow regimes at ecologically relevant velocities (44 and 64 cm/s). Our results indicate that size and flow both influence growth rates and that the 2 interact significantly. Young larvae exhibit faster growth rates and a greater positive response to increased flow speed. This result might help explain why smaller larvae have a greater propensity to disperse than larger larvae: the benefit of increased growth rate that they receive from relocating to faster flow might balance the risks inherent in dispersal.     

Retention mechanisms of white perch (Morone americana) and striped bass (Morone saxatilis) early-life stages in an estuarine turbidity maximum: an integrative fixed-location and mapping approach

The small‐scale distribution and retention mechanisms of white perch (Morone americana) and striped bass (M. saxatilis) early‐life stages were investigated in the upper Chesapeake Bay estuarine turbidity maximum (ETM). Physical measurements and biological collections were made at fixed‐location stations within the ETM during three research cruises in 1998 and two in 1999. Results were compared with mapping surveys of physical properties and organism distributions above, within, and below the ETM. Physical conditions at the fixed stations differed markedly among cruises and between years due to differences in freshwater flow and wind. In each year, striped bass and white perch larval concentrations were highest in waters of salinity 1–4. Larvae were more abundant in the ETM region in 1998, a high‐flow year, suggesting that the ETM provides favorable nursery habitat when low salinity waters and the ETM coincide in high freshwater‐flow conditions. In 1998, the earliest pelagic life stages of fish larvae (eggs, yolk‐sac larvae) and the copepod Eurytemora affinis, an important prey of feeding larvae, apparently were retained in deep, landward‐flowing water within the salt front and ETM region. Statistical analyses indicated that distributions of white perch and striped bass post‐yolk‐sac larvae were associated with E. affinis distributions and suggested that retention of larval fish could result from tracking prey. Comparing fixed‐station and mapping approaches demonstrates the importance of sampling at different spatial scales within the ETM region and suggests that larvae are faced with trade‐offs between selecting zones of high retention or high visual‐feeding success.     

Comparative digestive enzyme ontogeny in two marine larval fishes: Pacific threadfin (Polydactylus sexfilis) and bluefin trevally (Caranx melampygus)

The specific activity of digestive enzymes; aspartic and serine protease, collagenase, lipase, acid and alkaline amylase, acid and alkaline phosphatase, and chitinase was assayed throughout early development in two species of marine fishes: the Pacific threadfin (Polydactylus sexfilis) and bluefin trevally (Caranx melampygus). Specific enzyme activities were determined on whole larval extracts sampled at selected stages of development, from day 0 to day 30 post-hatching. Similar developmental patterns of enzyme specific activity were observed in the two species, although differences in timing, amplitude and effects of first feeding were noted. Amylase activity increased prior to first feeding, peaking at the middle of the larval period, and becoming nearly undetectable by the time of larval-to-juvenile metamorphosis. Serine protease, collagenase, lipase and alkaline (and acid for threadfin) phosphatase activities increased gradually, followed by sharp increases to a plateau during the second half of larval development. Aspartic protease and chitinase activities, in addition to acid phosphatase (for trevally), were low to undetectable in the first half of development, increasing through metamorphosis. In the trevally only, this group of enzymes exhibited high activity levels at the time of hatching, followed by a precipitous drop. Species-dependent differences in enzyme specific activity at first feeding may have been a result of differences in yolk utilization. These results, taken in the context of earlier reports, lead us to conclude that carbohydrate utilization may play a significant role in the earlier phases of development among some marine larvae, followed by a shift to protein and lipid utilization.     

A numerical analysis of population dynamics of the sand lance (Ammodytes personatus) in the eastern Seto Inland Sea, Japan

The Japanese sand lance (Ammodytes personatus) population in the Seto Inland Sea shows large fluctuations every few years. Its recruitment to the fishery resources is much more dependent on the survival rate of 0-age fish than is the case for other fish species because 0-age fish directly recruit to next-year spawning adults. Since the 0-age fish population depends on its larval population, which is easily influenced by natural and human impacts, high larval mortality due to predation by adults and high dispersion of larvae by wind-driven currents from spawning grounds to other locations can have very important effects on stock fluctuations. A numerical model of population dynamics of the sand lance in the eastern Seto Inland Sea was developed that focuses on wind-induced transport and adult predation of the larvae. The model successfully simulated the two-year periodic fluctuation and was consistent with actual fluctuations in fishery catches. It suggests that the various coefficients given in the model are appropriate to explain the actual population dynamics of the sand lance. The model shows that adult predation on the larvae plays a very important role in the two-year periodic fluctuation, and the maximum predation rate is estimated to be ten individuals per day. Wind-induced transport modifies the impact of this predation. Variances in the two-year periodic fluctuation become smaller with weaker westerly wind velocity. The appropriate transport rates from Bisan Strait and the Harima-nada Sea are estimated to be 0.04 d^-1 and 0.01 d^-1, respectively.