Monitoring of sea trout post-smolts

Sweep netting has been undertaken monthly in the Laxford and Polla estuaries since 1997. The length and weight of the fish are recorded, the fish tagged with a small visible implant tag behind the eye and the adipose fin removed (see pictures). Each tag has a unique code, so growth rates can be calculated if the fish is recaptured. The fish are also examined for sea lice and the lice numbers counted. Trends in the population can then be examined and the abundances compared to those on the neighbouring fish farms.

The location of the tag                    The adipose fin (shown) is removed

Results

The fish caught were of varied age (Fig. 1) and length (Fig. 2), reflecting a mixed population structure. The age structure in the three estuaries was similar, although the Kyle of Durness returned few adults (Fig. 1). From Fig. 1 the predominant smolt age in all rivers is 2 years (S2), although there was a number of S3’s also present. The length of fish in each estuary was similar although there was a greater proportion of larger fish in the Polla (Fig. 2), while post-smolts dominated the Kyle of Durness samples.

A proportion of the fish examined were from previous smolt runs (Fig. 1; Table 2). There does not appear to be a pattern in the proportion of post-smolts within the samples, but they dominated the catches in all estuaries after May.  While a May smolt run is normally found in West Sutherland (WSFT 2008), the Laxford smolts appear to have started migrating in April this year.

The presence of post-smolts at all sites throughout the year indicates a heavy usage of estuaries by this group, presumably for feeding and shelter. That the sea trout populations are relatively static can be inferred from the information on recaptures, where all tagged fish recaptured during 2008 were taken in the same location as originally tagged.

There were 12 recaptures during 2008, most within the estuary netting but one was captured by rod and line in the River Polla. The growth of recaptured trout is shown in Table 4a, for the Polla estuary, and Table 4b, for Laxford Bay. No recaptures were reported in the Kyle of Durness.  While the majority of the recaptures were tagged in 2008, 2 were originally tagged in 2006 and 1 in 2007. This gives yet more information on sustained growth rates and demonstrates the potential effectiveness of the tagging programme. All recaptured fish were taken in the same estuary as they were tagged demonstrating that most sea trout remain within a small area.

The average growth, per month, is 6.23 mm, and 8.35 g within the Laxford, which is similar to growth in 2007 Within the Polla average growth rates were 18.35 mm and 10.56 g, which shows a reduction in length growth but an increase in weight compared to 2007. This demonstrates a difference between the 2 populations, with Polla fish on this occasion showing a greater growth than those in the Laxford, similar to that observed in 2007 (WSFT 2008). While based on a small sample size, this indicates that a greater variability in growth rates exists within a system, and suggests that growth may not be the only factor influencing the variations reported in catches.

It would appear that while the growth rate in the Laxford continued to fall since 2006 the Polla growth rate has increased to a level greater than that observed since 2001.  In both estuaries there is a general trend of a high growth rate following a year with a low growth rate, a situation that is particularly obvious in the Laxford.  The years with lower growth rates in the Laxford tend to be the years with a higher growth rates in the Polla and visa versa. This demonstrates the complexity of trout population dynamics and the interactions with external factors, such as food supply and temperature.

 The lengths and weights of recaptured trout within the Polla estuary

^*Caught by rod and line

The lengths and weights of recaptured trout within Laxford Bay

Showing the average SGR for fish within the Laxford and Polla estuaries, by year

Sea Lice Infestations

Sea lice were present to a variable degree throughout the year in all estuaries, although more prevalent within the Laxford. Lice were found during each sampling occasion. Chalimus stages dominated the samples every month, with gravid females only appearing in small numbers.  Lice numbers were generally high in both the Laxford and Polla, peaking in both estuaries in June. Numbers increased with time in the Kyle of Durness but remained relatively low. 

Laxford

The pattern of lice abundance within the Laxford samples  is similar to that in previous years, rising through the year until June, before dropping again (Marshall 2003; WSFT 2008). The low abundance in May is likely to reflect a high number of fresh run smolts in the sample. Higher median values in April and June would indicate that there were a larger number of more heavily infested fish within those samples compared to previous years. Lice numbers on neighbouring cages were low and no correlation can be seen between them and those found on wild fish. A few gravid lice were observed in July and August, but the catches remained dominated by Chalimus and mobile staged.

Polla

The abundance of lice is similar to that observed in the Laxford, with an increasing lice population until June before it starts to decline again. However, the values were significantly lower than those observed within the Laxford.  There were a greater proportion of adult and gravid lice within the Polla catch compared to the Laxford, with gravids observed each month. This indicates a mature lice population and is frequently observed within the Polla suggesting that there is a lice population being maintained within the wild fish population in Loch Eriboll. Lice numbers on neighbouring cages were very low and no correlation can be seen between them and those found on wild fish.

Kyle of Durness

The Kyle of Durness is the furthest sea loch from fish farm cages within the west Sutherland area and as such is a suitable control site for this monitoring programme. The pattern of lice abundance observed within the Kyle of Durness was unique to the area as it continued to increase with time. In addition, the maturation of lice was observed from mobile to gravids, with no Chalimus seen during this sampling period. These facts, coupled with the increasing numbers of larger fish observed moving in the Kyle over the monitoring period, would suggest that the lice population on sea trout post-smolts are the result of lice breeding on the adult, wild salmonid population.

 The exceptionally dry summer, and therefore low river flows, and high temperatures have created ideal conditions for lice development. A lack of freshwater gives maximum survival for the different lice stages, while the inability of returning adults to ascend into the rivers has maintained the population of adult and gravid lice. While returning salmonids routinely have high densities of adult sea lice, these die on migration of the salmonid into the rivers and are lost to the breeding population. This did not happen during 2008, when salmon were held in the estuaries for long periods of time, a situation noted in various places including the Aberdeenshire Dee (M. Bilsby, pers. comm.).

The Laxford and Polla, both estuaries containing salmon cages in their second year of production, showed similar patterns of lice abundance, although the Laxford trout were significantly more heavily infested. This contrasted to the pattern observed in the control site and indicates that the presence of cages influences lice distribution. However additional monitoring of the Kyle will be required to confirm, particularly given the unique weather conditions observed during 2008.

 In addition, a study of the louse response to Slice within the Laxford population undertaken by Loch Duart Ltd suggested that the lice on the wild sea trout are not derived from the cages within Loch Laxford. Analyses carried out by the Fish Vet Group confirmed that the wild population of lice are naive to Slice, while those obtained from the cages have some ‘knowledge’ of the treatment. This creates an anomaly that will require additional analysis to assess.

References

Butler, J.R.A. (2002). Salmonids and sea louse infestations on the west coast of Scotland: sources of infection and implications for the management of marine salmon farms. Pest Mgmt. Sci. 58: 595 – 608.

 McLellan, A. (2008). Monitoring of sea trout for sea lice infestations, West Sutherland and Wester Ross 2008. www.tripartiteworkinggroup.com.

 Marshall, S. (2003). Incidence of sea lice infestations on wild sea trout compared to farmed salmon. Bull. Eur. Ass. Fish Pathol. 23(2):  72 – 79.

 WSFT (2008). Monitoring of sea trout post-smolts, 2007. Unpubl. Report to the West Sutherland Fisheries Trust, Report No. WSFT2/08.

How can you help?

All fish that are tagged have had the adipose fin, the small fleshy fin behind the tail, removed. Should you catch a sea trout without this fin, please look behind the eye and note the colour and number of the tag. An estimate of length and weight should then be made (if the fish is being returned) and the information, together with date and place of capture, given to the Biologist.

Thanks must be given to the many people who assisted with the sampling over the past year and without whom the project could not have been completed, particularly the proprietors and staff of Reay Forest and Rispond Estates. This project has received funding from the Crown Estate Commission.