ICES Journal of Marine Science: Journal du Conseil

MacKenzie, K.; Longshaw, M.; Begg, G. S.; McVicar, A. H.

doi: 10.1006/jmsc.1997.0362pmid: N/A

Lepeophtheirus salmonis infection occurred on 30% of 622 sea trout examined from Scottish waters, with intensities up to 258 per fish. Caligus elongatus was rarer, being recorded on only 3% of trout, at a maximum intensity of 14. Geographical and temporal variations were found in the pattern of infection with L. salmonis. Most lice recorded were chalimus stages. On each fish, the same developmental stage of lice predominated, but this stage varied, even between fish caught on the same location at the same date. Haemorrhage at the base of dorsal fins infected with chalimus infection was the only skin damage found associated with lice. Analysis of the population structure of lice at two locations showed a progression of developmental stages with time, but also evidence of continual re-infection up to the end of sampling in late July. A positive relationship was found between sea trout condition and the intensity of infection with chalimus III+ larvae but a negative relationship was found with the intensity of chalimus II− infection, possibly related to the length of time in, and adaptation to, sea water. The heaviest infection of sea trout with lice was recorded in the salmon farming areas of the west and north-west of Scotland. However, no clear pattern emerged with levels of infection at single locations in relation to the proximity to the nearest salmon farm or with the pattern of infection (and larval production) within an adjacent salmon farm.

Schram, T. A.; Knutsen, J. A.; Heuch, P. A.; Mo, T. A.

doi: 10.1006/jmsc.1997.0357pmid: N/A

The occurrence of the parasitic copepod Lepeophtheirus salmonis Krøyer Caligus elongatus Nordmann on sea trout Salmo trutta L. was investigated in the Arendal archipelago, southern Norway, an area without salmon farms. The occurrence of both parasite species followed a seasonal pattern. Prevalence (percent infested fish) and median intensity (median number of lice on the infested fish) of both species increased in the spring, and declined in the winter. A prevalence of 100% was reached in 1993, 1994 and 1995 for L. salmonis, while the prevalence of C. elongatus generally peaked at about 90%. In March, only adult L. salmonis were found, and small numbers of pre-adults appeared in April. The proportion of chalimi of L. salmonis never reached more than 15% each month of the study. The infected fish were without wounds and apparently in good condition. Neither the condition factor of the host, nor host age were correlated with the intensity of L. salmonis or C. elongatus.

Mo, T. A.; Heuch, P. A.

doi: 10.1006/jmsc.1997.0359pmid: N/A

The occurrence of the parasitic copepod Lepeophtheirus salmonis Krøyer on sea trout Salmo trutta L. was studied in the Askerelva river and in the adjacent Oslo Fjord, south-eastern Norway, where there are no salmon farms. Sampling was carried out in 1992 and 1993. Fishing was started in May, but no sea trout were caught in the river before August. Maximum prevalence (percent infested fish) in the monthly samples from the river was 50%, and the mean intensity (mean number of lice on the infested fish) in all river samples pooled was 19.9 parasites. Most were in the pre-adult or adult stage. The fish from the fjord were larger and had, in general, fewer lice than fish from the river (pooled mean intensity=7.6 lice), but the difference in mean intensity of infestation was not significantly different. There was no correlation between condition factor and the intensity of infestation on trout from the river nor from the fjord.

Costelloe, M.; Costelloe, J.; Coghlan, N.; O'Donohoe, G.; O'Connor, B.

doi: 10.1006/jmsc.1997.0360pmid: N/A

The distribution of Lepeophtheirus salmonis larvae was investigated by taking plankton tows from stations on a regular basis in a number of bays on the west coast of Ireland during the spring/summer period in 1995 and 1996. Larvae were only found consistently at a station close to a salmon farm at the mouth of Killary Harbour. In the inner harbour, close to the mouths of two rivers and distant from the farm, larvae were recovered sporadically. Similar sporadic occurrences were recorded in two other estuaries in which no farms were located. Highest densities of larvae were found in the spring of both years in the estuarine areas with a subsequent decrease in the following months.

MacKinnon, B. M.

doi: 10.1006/jmsc.1997.0361pmid: N/A

An overview is presented on the interactive parameters that influence fish susceptibility to infection. In particular, the importance of genetically determined resistance, stress, immunocompetency, and nutrition are discussed in relation to their influence on susceptibility of salmonids to infection with sea lice. It is suggested that these factors should be taken into account, together with other factors that determine infection intensity, such as the source and number of infective stages of sea lice, when devising programs for the management of wild and aquacultured salmonids.

Dawson, L. H. J.

doi: 10.1006/jmsc.1997.0358pmid: N/A

The physiological impact and pathological effect of salmon lice Lepeophtheirus salmonis (Krøyer, 1837), on wild sea trout Salmo trutta L., was investigated in 10 rivers in Western Ireland. No skin damage was apparent on fish with mobile pre-adult and adult lice, but severe fin erosion was recorded in many of the fish that were infected with chalimus stages. The attached copepodid and chalimus stages of the parasite did not reduce the physiological performance of the sea trout although there was a significant reduction in total protein, serum albumin and cholesterol in fish with mobile stages compared with fish with attached stages only and fish with no lice. This may reflect the feeding history of the fish, as most of the fish with mobile lice were from the same area, or the physiological effects of lice infections may be stage-specific.

Hayes, K. R.

doi: 10.1006/jmsc.1997.0342pmid: N/A

An ecological risk assessment methodology is suggested as a means to investigate the efficacy of ballast water management strategies, addressing any one of a number of potential endpoints linked to the transport, release and control of non-indigenous marine organisms associated with ballast water. The methodology is based upon the Quantitative Risk Assessment paradigm more commonly employed in nuclear and chemical process industry. The paper advocates a case-specific approach to ballast water risk assessment and provides a general discusion of some of the difficulties that may be faced in such an assessment, illustrated by reference to a hypothetical biocide treatment strategy.

Marchal, P.; Horwood, J.

doi: 10.1006/jmsc.1997.9994pmid: N/A

This study proposes a dynamic control of fisheries by fishing effort, calculated so as to optimize a novel cost function, over a long period of management, based upon an example of the exploitation of the Celtic Sea cod. This cost function is a flexible weighed compromise between: (i) minimizing the variability in fishing effort; (ii) minimizing the variability in yields; (iii) achieving an agreed fishing effort target; and (iv) achieving an agreed yield target. Increasing the weight on the criteria (ii) and (iii), and decreasing the weight on the criteria (i) and (iv), results in a more rapid movement of fishing effort towards the agreed long-term target, a decrease in short-term yields, and an increase in long-term yields. When the weight attached to (iii) is non-zero, the optimization of the cost function provides a non-oscillatory, convergent, and near-linear control. When the weight attached to (iii) is zero, the optimization of the cost function usually results in a non-linear divergent control, which is an artefact related to the choice of the length of the management period.

Secor, D. H.; Houde, E. D.

doi: 10.1006/jmsc.1997.9996pmid: N/A

After the collapse of striped bass Morone saxatilis stocks in the late 1970s, hatchery programs and release experiments were instituted to evaluate the potential to restore striped bass in Chesapeake Bay. Because survival of striped bass larvae to first feeding (7 d after hatch) is low in Chesapeake tributaries, ranging from 0.2 to 5.2% of eggs spawned, it is possible to enhance survival through hatchery propagation of eggs and yolk-sac larvae, and it may be feasible to supplement recruitment by stocking post-yolk-sac larvae. During 1991–1993, otoliths of 31.7 million hatchery-produced striped bass larvae (5–13 d after hatch) were chemically marked and released into two tributaries of Chesapeake Bay. In years of moderate to poor natural larval production (1991, 1992), stocked larvae contributed 20 to 30% to overall juvenile abundance. In 1993, a year of high natural production, stocked larvae contributed only 5% to juvenile abundance, although numbers contributed were higher than in previous years. Using field and hatchery estimates of larval and juvenile growth and mortality, enhancement strategies were compared in which fish were released at three different ages: larvae (7 d post-hatch), summer-stocked juveniles (55 d post-hatch), or fall-stocked juveniles (220 d post-hatch). Based upon hatchery records, reared larvae and juveniles experienced substantially higher rates of growth and survival than did wild larvae and juveniles of similar age. In years of low natural egg production, cohort biomass at 220 d post-hatch was highest when juveniles were stocked in summer or fall. When approximate costs were incorporated into the analysis, the efficacy of stocking 7-d-old larvae was higher than summer and fall stocking of juveniles if stocked larvae encountered favourable conditions. In years of poor natural recruitment, stocking post yolk-sac larvae into estuarine tributaries could supplement stocks of striped bass and possibly other anadromous species which experience high embryo and yolk-sac larva mortality.

Lipinski, M. R.; Durholtz, M. D.; Underhill, L. G.

doi: 10.1006/jmsc.1997.0274pmid: N/A

This paper describes the first successful field tag-recapture experiment, aimed at validating daily increments in the statoliths of chokka squid (Loligo vulgaris reynaudii). Seven hundred and forty-two squid were tagged and injected with oxytetracycline (OTC) and 63 were subsequently recovered after 4–20 days spent free in the sea. Forty-seven statoliths were successfully prepared for increment reading, eight of which satisfied stringent requirements for validation: good visibility of increments, sharp definition of the OTC band and a clear, undamaged margin. These eight statoliths yielded results indicating that the increments in chokka males in the size range 290–370 mm mantle length (ML) were deposited daily and that the same was true for a 173 mm ML female. The results of an analysis of the errors involved in the increment recognition stage (counting) do show, however, that proper statolith preparation, increment identification and subsequent reading are crucial for obtaining accurate age estimates. The error in counting increments in a poorly prepared statolith may be three times higher than the error for a well prepared statolith in the technique used, which is felt to be one of the best presently available.This technique relies on the grinding of both sides of the statolith in the frontal plane (contrary to the most frequently used transverse sections). Precise position of the grinding plane is determined and maintained during the procedure.