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 largest catch within a single sweep was 117 fish in the Polla estuary during September (Table 1). A comparison of the catches with time in both estuaries demonstrates the variability in the abundance of fish within the sample sites and the difficulties in using these results to demonstrate population size.  The by-catch from the netting in both estuaries was as expected from previous years, with few species and low numbers observed. There were large numbers of salmon smolts and adults captured in 2009 compared to previous years.

 The fish caught were of varied age and length, reflecting a mixed population structure. The age structure in the three estuaries was similar, although the Kyle of Durness returned only one adult. The predominant smolt age in all rivers is 2 years (S2), although there was a number of S3’s also present. S1’s were also observed in the Laxford and Polla. The length of fish in each estuary was similar although post-smolts dominated the Kyle of Durness samples.

 A proportion of the fish examined were from previous smolt runs. 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 and Kyle smolts appear to have started migrating in April, a pattern also observed in the Laxford in 2008.

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 bar one of the tagged fish recaptured during 2009 were taken in the same location as originally tagged. The exception in this case was a sea trout tagged in the Kyle of Durness and recaptured in the Polla.

There were 7 recaptures during 2009, all bar within the estuary netting. The exception was taken during broodstock netting in the Laxford. The growth of recaptured trout is shown. Of the recaptured, 2 were originally tagged in 2007 and 3 in 2008. This gives yet more information on sustained growth rates and demonstrates the potential effectiveness of the tagging programme. All bar one of the recaptured fish were taken in the same system as they were tagged demonstrating that most sea trout remain within a small area.

The average growth, per month, is 9.4 mm, and 8.9 g within the Laxford, which is a lower length increase but similar weight increase to that found in 2008. Within the Polla average growth rates were 11.3 mm and 33.9 g, which shows a reduction in length growth but an increase in weight compared to 2008. The Kyle of Durness gave a growth of 7.3 mm and 7.2 g per month respectively. However it should be noted that growth rates in the Polla and Kyle of Durness are based on one recapture only.

 It would appear that the growth rates in the Laxford and Polla continue to show opposite patterns, with a rise in the Laxford mirrored by a fall in the Polla. This follows the trends seen over the 11 years of the study. The growth rate within the Laxford remains low, while that in the Polla is about average for the estuary. This demonstrates the complexity of trout population dynamics and the interactions with external factors, such as food supply and temperature. The Kyle of Durness has a similar growth rate to that of the Laxford.

 The lengths and weights of recaptured trout within the Polla estuary

^*tagged in the Kyle of Durness

  The lengths and weights of recaptured trout within Laxford Bay

⁺caught during broodstock netting

 The lengths and weights of recaptured trout within the Kyle of Durness

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

Sea Lice Infestations

Sea lice were present to a variable degree throughout the year in all estuaries, although more prevalent within the Polla. Lice were found during each sampling occasion, with the exception of the Kyle of Durness in April. Chalimus stages only dominated the catches in the Laxford in April, with mobile stages being more common. Gravid females were present on all occasions from July on, and in the Polla every month. Lice numbers were generally high in all estuaries, although the Laxford densities were low from June onwards. Numbers increased with time in both the Kyle of Durness and the Polla. 

Laxford

Lice abundance within the Laxford samples declines through the year, with the exception of a peak in July. The neighbouring cages were fallowed in May 2009 with the removal of the broodstock fish. No fish were on site over the sampling period. All samples were dominated by juvenile lice, until September when only adults were observed. An ageing of lice from Chalimus to gravid could be observed throughout the Laxford samples.

Polla

The abundance of lice shows an increasing lice population which drops slightly at the end of the year. Median values indicate that, with the exception of June and September, however this is likely to have been caused by a few more heavily infested individuals. Chalimus stages were rare within the samples, with gravid females recorded 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 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. As with the Polla, lice levels within the Kyle of Durness continued to increase over the sampling period. In addition, the maturation of lice was observed from Chalimus, through mobile to gravid females, suggesting an immobile host population.

2009 was a year of extremes, with exceptionally warm, dry conditions in June and July, followed by wet conditions in subsequent months. This will have created ideal conditions for lice development at the time of the smolt run. In addition, the sea trout run appeared to have been delayed throughout the area. Anecdotal evidence suggesting that, for the Polla, this delay was approximately 2 months in length. Thus fish were being retained within the estuary, creating an additional host population for the lice. Similarly, salmon catches within the area were mixed, indicating that mature salmon may also be sitting in the coastal areas.

Laxford and Polla lice data, historical analysis

The counting of sea lice on wild sea trout caught within the Laxford started in 1997, while Polla counts began in 1999. By showing the differences in prevalence and abundance with time, trends in the data can be assessed. In addition, there is evidence from many areas that the abundance of lice on wild fish is related to the year of production on the farm, being higher in the second than the first year. This pattern is also widely accepted within the farms themselves.

It would appear that the prevalence of lice on fish is affected by the month, with each year showing a similar pattern. In general the percentage of trout with sea lice starts low, increasing to June before decreasing again. There is no relationship between the prevalence of lice and the stage of production within the neighbouring farm in either the Laxford or the Polla.

 Abundance of lice on the wild fish appears to have a more recognisable pattern, with higher abundance apparent in the 2^nd year of production within both the Laxford and Polla samples. However analysis of the data over the period of the study shows that this relationship, while statistically significant in the Polla is not significant in the Laxford.

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.

 Hunter, D-C. (2010). Monitoring of sea trout for sea lice infestations, West Sutherland and Wester Ross 2009. 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.