Wild Sea Trout Research Articles

Genetic differentiation of populations of the copepod sea louse Lepeophtheirus salmonis (Krøyer) ectoparasitic on wild and farmed salmonids around the coasts of Scotland: Evidence from RAPD markers

Sea trout are the sea-going migratory form of the freshwater brown trout ( Salmo trutta L.) and since 1989 there have been marked declines in their stocks on the west coasts both of Scotland and Ireland. Various factors have been attributed as possible causal agents in these stock declines, including fresh water acidification, overfishing, climatic fluctuations, habitat degradation and sea lice parasitic burdens. The putative impact of infestations of sea trout by the ectoparasitic copepod sea louse, Lepeophtheirus salmonis (Krøyer), has featured prominently in the controversy, especially with regard to the role of inshore commercial salmon farms as a possible source of infestation of wild salmonids by sea lice. This study focused on the population genetics of L. salmonis around the coasts of Scotland: We sampled fish from wild and cultured stocks and included salmon ( Salmo salar L.), rainbow trout ( Oncorhynchus mykiss Walbaum) and sea trout as host species. Analyses of allozyme variation of sea lice were confined to data for two polymorphic loci ( Fum, Got-2) and conformed to our initial expectation — that the inclusion of a planktonic larval phase in the life cycle of the copepod, in addition to the high mobility of the host fish, would enhance gene flow and preclude genetic differentiation of L. salmonis populations as a result of random drift alone. DNA polymorphism was quantified by means of PCR and RAPD analysis. Six primers were screened for 16 samples (from wild and farmed salmon, wild sea trout and farmed rainbow trout) — including the east, north and west coasts of Scotland — and the data analyzed by AMOVA (Analysis of Molecular Variance). In contrast to the allozyme results, the RAPD analysis showed striking patterns of genetic differentiation around the coasts of Scotland. The overall pattern was one of genetic homogeneity of L. salmonis populations sampled from wild salmon and sea trout. All of the L. salmonis samples taken from farmed salmon and rainbow trout did, however, show highly significant levels of genetic differentiation, both between wild and farmed salmonids and among the various farms themselves. Evidence of high levels of small-scale spatial or temporal heterogeneity of RAPD marker band frequencies was shown for the one farm from which repeat samples (July and November, 1995) were analysed. Samples of sea lice taken from west coast wild sea trout subjected to RAPD analysis also revealed the occurrence of putative “farm markers” in some individual parasites, indicating that they had possibly originated from salmon farms.

Artificial reconditioning of wild sea trout, Salmo trutta L., as an enhancement option: initial results on growth and spawning success

Abstract Following a sea trout stock collapse in 1989 and 1990, wild sea trout kelts were successfully reconditioned in brackish water using semi‐moist pellets and a commercial broodstock diet. The on‐growing of wild sea trout smolts was not so successful. Natural mortality was 12.9%, and 36.1% of the fish failed to adapt to the artificial diets: 82% of these were smolts. Instantaneous growth was highest from June to August (0.74%), with an overall rate for April to September of 0.51%. At the end of the first year. 36 females and 26 males matured; 34 females, with a mean fork length of 33.2 cm, were stripped yielding 22860 ova at a mean rate of 682 ova per female or 1586 ova kg‐1 body weight. The survival of green ova to eyed ova was 90–98%, and survival from green ova to hatched alevin was 95%.

The movement of wild sea trout, Salmo trutta L., smolts through a river estuary

Abstract The movement of sea trout. Salmo trutta L., smolts through the estuary of the River Avon, southern England was studied using miniature 300 kHz acoustic transmitters and an array of strategically placed acoustic sonar buoys. Sea trout smolts were trapped 3.25 km above the head of tide as they moved downstream and the transmitters were surgically implanted into the peritoneal cavity of the fish. Subsequent movement of the smolts through the estuary was predominantly nocturnal and occurred mainly during an ebb tide or the period of slack water between two high waters. During the hours of darkness the ground speed of the smolts through the estuary was similar to that of passively drifting objects. However, where movement did occur during daylight hours ground speeds of smolts were significantly slower. Vertical salinity profile measurements of the estuary indicated that individual smolts may have experienced salinities ranging from fresh to 100% sea water during their migration. The behaviour of the smolts is discussed in relation to the possible environmental and physiological cues controlling the estuarine migration.

Production of nauplii of Lepeophtheirus salmonis (Krøyer) (Copepoda: Caligidae) from farmed and wild salmon and its relation to the infestation of wild sea trout ( Salmo trutta L.) off the west coast of Ireland in 1991

The number and fecundity of ovigerous Lepeophtheirus salmonis (Krøyer) infesting wild and farmed salmon and daily production, from these fish, of nauplius I of L. salmonis was estimated for a number of areas off the west coast of Ireland between March and July 1991. Significant differences in body size and fecundity of the parasite were apparent both over time and among sampling sites. Body size and egg number were positively correlated. Lice infesting wild salmon were larger and carried approximately twice as many eggs as those infesting farmed salmon. Farmed salmon contributed 95% of the total production of nauplius I of L. salmonis in the mid west coast region. Production ranged from 0 to 3.8 × 10 7 nauplii per day from salmon farms in individual embayments. Using data from six embayments the production of naupliusI during mid April was correlated with parasitic intensity of L. salmonis infesting wild sea trout ( Salmo trutta L.) 3 weeks later. Although the production of nauplii remained relatively constant between March and July, transmission to sea trout was apparently restricted in time to late April and May. Temporally varying susceptibility to infestation may therefore have a role in determining the level of infestation that develops.

Diseases of wild sea trout in Scotland in relation to fish population decline

Sea trout cathceshave been unusually low in recent years in some areas of Scotland, particularly parts of the west coast, coincident with a decline in sea trout catches and stocks in parts of western Ireland. The causes of the widespread decline are at present unknown and are being investigated but climatic as well as biological influences are likely to be involved. A wide variety of pathologies has been detected in sampled sea trout, particularly in the kidney, liver, and heart but with no overall consistency in occurrence between areas or within individual rivers. In some fish the lesions were sufficiently extensive to be considered to be possibly threatening to organ function, but no correlation was established between severity and loss of fish condition. Juvenile trout sampled in seawater, in estuaries and lower rivers were mainly in good condition. However, some obtained in rivers in northwestern Scotland during early summer had cranial lesions and dorsal fin damage associated with sea lice. The prevalence and intensity of infection varied between and within sites with high intensities being found on both west and east coast rivers. There was no clear relationship between lice burden and the extent of fin damage on individual fish. Samples obtained in the late summer and autumn were clear of lice and showed no lesions. A wide range of other metazoan parasites was recorded, with no indication of correlation with host condition. No significant bacterial infection was detected, but infectious pancreatic necrosis (IPN) virus was detected in fish from two west coast rivers. The significance of the pathologies and occurrence of metazoan parasite and other infections is being studied but with the data currently available no cause-effect relationship between disease and decline of sea trout populations could be established.

Optimal Time and Size for Smolt Migration in Wild Sea Trout (Salmo trutta)

A tagging programme, in which wild sea trout (Salmo trutta) were marked at the parr stage with internal tags in winter and recaptured during smolt migration in a trap in the following spring, was conducted for three seasons in a small stream in southwestern Sweden, individuals that were initially smaller migrated later, grew more in length, and were smaller at migration than those initially bigger. Body length at migration decreased with migration time during the season for tagged as well as nontagged trout. A model for optimal time of and size at migration is proposed, based on the assumption that the expression (body length at the end of the season) × (survival over the season) is maximized and that migratory and postmigratory survival is positively size dependent. The qualitative predictions on the relationship between initial size and time of and size at migration are supported by the tagging data. Pre- and postmigratory growth rates are predicted to have opposite effects on migration time, so the migration timing is expected to depend strongly on the degree of dependence between these.

Early sexual maturation in males of wild sea trout, Salmo trutta L., inhibits smoltification

Precocious maturation among wild sea trout, Salmo trutta (L.), was studied in two small streams in south‐western Sweden. The proportion of parr males varied between 17.9 and 57.0%. Parr males and immature individuals of the same age were compared with respect to length distribution, migratory tendency and seawater tolerance. Precocious males were, on average, longer than immature fish. Marking and recapture showed a reduced or inhibited migration tendency of parr males. The seawater tolerance of these was also consistently lower than for juveniles. The seasonal pattern in osmoregulatory ability was similar for both categories, with a peak during the smolt run.