Monitoring of sea trout post-smolts, 2013

Transcription

1 Monitoring of sea trout post-smolts, 213 A report to the West Sutherland Fisheries Trust, Report No. WSFT2/14 January 214 Shona Marshall Fisheries Biologist West Sutherland Fisheries Trust Gardeners Cottage Scourie By Lairg Sutherland IV27 4SX

2 Monitoring of sea trout post-smolts, 213 Introduction Started in 1997, this project has enabled the establishment of a good database of the population dynamics of sea trout within the area. Additional information about lice burdens on the trout within the estuaries has also provided an analysis of the relationship between fish farms and sea trout, with particular regard to sea lice (Marshall 23; WSFT 213). The monitoring of post-smolts was originally designed to give an indication of the migrations and growth of sea trout within the area. The individual tagging of fish, combined with the measurements taken at capture, gave a baseline from which to assess these parameters following re-capture by nets or rod and line. In addition to these data, the numbers of sea lice were also assessed. This has now progressed, such that sea lice counts are the main part of the project, with the tagging of fish giving additional information. Materials & Methods Two estuaries, Laxford Bay and the Polla estuary, were sampled monthly where possible from March to October, at low tide. Sampling was performed using a 5 m sweep net with a stretched mesh size of 15 mm hand pulled in a large circle to give one sweep of the area. Differences between the number examined and tagged (Table 1) reflect the presence of recaptures, the small size of trout involved or difficulties in loading the injector. Where trout <15 cm are involved, injection of the tags can prove difficult with only a thin membrane available to hold the tag and is therefore not undertaken. In addition, the Kyle of Durness and Kinloch River were sampled on numerous occasions between May and July as part of the RAFTS Managing Interactions Project ( These fish were not tagged. All sea trout were removed and anaesthetised with 2-Phenoxyethanol. The length (± 1 mm) and weight (± 1 g) were recorded, scales removed and a visible implant (VI) tag implanted behind the eye. The fish were examined for the presence of sea lice, which were counted and roughly staged, i.e. Chalimus, mobile, adult and gravid female. The condition index for the trout was calculated from the length and weight such that: Condition Index = 1W/L 3, where weight is in grams and length in cm. Throughout this document, post-smolts are defined as fish that went to sea in this year. Adults refer to fish that have had one year or more at sea. The Specific Growth Rate (SGR) was calculated for the recaptured fish to give annual variations, such that: SGR = (((ln(final wt) ln(initial wt))*1)/time), where weight is in grams and time in days. Results and Discussion The largest catch within a single sweep was 89 fish in the Polla estuary during May (Table 1). A comparison of the catches with time in all 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. 1

3 Table 1 The number of fish examined and tagged, by estuary and month Laxford Bay Polla estuary Month No. examined No. tagged No. examined No. tagged March April May June * July 28 2 * August September October ( * 2 sweeps) Age, Length, Weight and Condition of Fish Captured The fish caught were of varied age (Fig. 1) and length (Fig. 2), reflecting a mixed population structure. The age structure in the two estuaries was similar, with the Polla returning the oldest adult (Fig. 1). From Fig. 1 the predominant smolt age in the rivers is 2 years (S2), although there were a number of S3 s also present. S1 s were also observed in small numbers in both of the estuaries. The length distribution of fish within the estuaries was different (Fig. 2), with the Polla having smaller post-smolts than the Laxford, but also a greater number of larger sea trout. A proportion of the fish examined were from previous smolt runs, although the Laxford catch was primarily post-smolt throughout the year (Fig. 1; Table 2). The Polla catches appear to show an increasing proportion of post-smolts with time. While a May smolt run is normal for the Sutherland area (WSFT 213), there were a large proportion of smolts taken in the March samples indicating that some smolts may have run earlier. This was most obvious within the Laxford. Table 2 The percentage of smolts within the catch Month Laxford Polla estuary Bay March 1 5 April - - May 1 65 June 96 - July 8 87 August 1 - September - - October 1 - The presence of post-smolts at both 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 of the tagged fish recaptured during 213 were taken in the same location as originally tagged (Badna Bay and the Laxford sharing an estuary). This confirms findings from previous years (WSFT 213). The mean length, weight and condition index, ± s.d., of post smolts per month are given in Table 3a for Laxford Bay and Table 3b for the Polla estuary. Condition index in the Laxford in March is very low, but improves in May and continues to increase slightly with time until July, before declining again. There were, unfortunately, few records from the Polla but there is still an increase with time. Length appears to vary with time, although no discernible pattern can be seen. This reflects the movement of post-smolts within the estuaries for feeding and shelter, and the movement of sea trout between marine feeding areas and the river. 2

4 6 5 4 No. Fish 3 2 Polla Laxford Age Fig. 1. The number of fish of each age taken in the estuaries 25 2 No. Fish Length (to lower cm) Fig. 2 The number of fish of each length taken in the estuaries Table 3a The mean length, weight, and condition index of the post-smolts in Laxford Bay, per month Month Mean length (± s.d.) (mm) Mean weight (± s.d.) (g) Mean Condition Index (± s.d.) March 182. ± ± ±.7 April May ± ± ±.12 June ± ± ±.18 July 29. ± ± ±.44 August 24.5 ± ± ±.7 September October

5 Table 3b The mean length, weight, and condition index of the post-smolts in Polla estuary, per month Month Mean length (± s.d.) (mm) Mean weight (± s.d.) (g) Mean Condition Index (± s.d.) March April May ± ± ±.22 June July ± ± ±.2 August September October Recaptures There were 9 recaptures during 213, 7 within the estuary netting, 1 in the smolt trap and 1 by rod and line. One fish from the Polla was re-captured twice. The growth of recaptured trout is shown in Table 4a for Laxford Bay and Table 4b for the Polla estuary. Of the recaptured trout, 2 were originally tagged in 212, 1 (J77) in 25, the rest in 213. This gives yet more information on sustained growth rates and demonstrates the potential effectiveness of the tagging programme. In particular, J77 gives significant information on the spawning patterns and survival of trout. As Badna Bay and the Laxford share an estuary at the location of the netting station, all of the recaptured fish were taken from the area of tagging. This pattern is common to the sampling programme over the past 16 years and demonstrates that the majority of sea trout do not stray far from their home rivers. Average growth rates within the Laxford were 3. mm, and 7.18 g per month, which is similar to that found in 211. Within the Polla average growth rates were 13.6 mm and -8.8 g, which show a significant decrease in both length and weight growth compared to 212. However, when the one fish that lost weight is removed from the equation growth was g, still lower than that recorded in 212 but reasonable. Figure 3 shows that the specific growth rates (SGR) in the Laxford remain low compared to previous years, although an increase has been recorded over the past 3 years. While the low SGR is of some concern, and will impact on the sea trout population, the increase is steady and bycatch has indicated the presence of prey species. Within the Polla calculations, only fish with a positive SGR were used, giving a low SGR, similar to that recorded in 211. The results demonstrate the complexity of trout population dynamics and the interactions with external factors, such as food supply and temperature. Table 4a The lengths and weights of recaptured trout within Laxford Bay Tag Tagged Recaptured Difference number Date mths * F82 Length (mm) Weight (g) Date mths + G38 Length (mm) Weight (g) Date mths # G25 Length (mm) Weight (g) * Captured in the Badna Bay smolt trap; + Tagged in Badna Bay; # Caught by rod and line in Badna Bay 4

6 Table 4b The lengths and weights of recaptured trout within the Polla estuary Tag Tagged Recaptured Difference Recaptured (2) Difference (2) number Date yrs J77 Length (mm) Weight (g) Date mths mths F47 Length (mm) Weight (g) Date days H11 Length (mm) Weight (g) Date days H13 Length (mm) Weight (g) Date days H16 Length (mm) Weight (g) Average SGR (mm/day) Year Laxford Polla Fig. 3 Showing the average SGR for fish within the Laxford and Polla estuaries, by year Sea Lice Infestations Sea lice were present to a varying degree throughout the year in both estuaries (Table 5), with lice found during all sampling occasions except the Laxford in March and May. There was a mixture of lice stages noted in the Laxford in June and July, although 99% of the July sample was comprised of Chalimus. Only Chalimus were found during August and October (Fig. 4a). The Polla samples demonstrated a maturation of lice with time (Fig. 4b), although all stages were present throughout the year. Lice numbers were high until August, when the Laxford population appeared to crash. The last sample was taken from the Polla in July. Total lice number per sample is, however, dependent on sample size and the use of abundance and intensity data give a better assessment of the situation. Table 5 The percentage of sea trout with the salmon louse, by estuary and month Month Laxford Bay Polla estuary March 1 April - - May 17 June 7 - July August 5 - September - - October 1-5

7 % lice March April May June July Month Fig. 4a Showing the proportion of each stage of lice within the Laxford samples, by month. The total number of lice is given at the top % lice Month Fig. 4b Showing the proportion of each stage of lice within the Polla samples, by month. The total number of lice is given at the top August In order to determine the potential impacts of sea lice on fish it is important to know the number of lice present per fish as well as their occurrence (Tables 6 (Laxford) & 7 (Polla)). The use of intensity will give a more accurate impression of the degree of infestation, being the number of lice on the infected fish, but abundance gives a better impression of the lice within the population. In addition, abundance is used in several studies, including Butler (22), and is the preferred method of recording within the neighbouring farms and is therefore given here. The use of the median value, being the middle value if they are ranked numerically, also gives an indication of the degree of infestation within the population, while removing the bias created by a single heavily infected individual. Laxford Lice abundance within the Laxford samples remained variable throughout the year, with lice numbers increasing to July, before declining (Table 6). Chalimus dominated all samples with lice with a mix of stages found only in June and July, although July had 99% Chalimus (Fig. 4a). This would suggest a September October March April May June July August September October Gravid female Adult Mobile Chalimus Gravid female Adult Mobile Chalimus 6

8 constant re-infection pressure on the fish. Gravids were only present in July, at very low density. It should be noted that the high abundance in October is based on one fish. Caligus were only recorded in the June samples, with 27 individuals noted within the population. The neighbouring cages were re-stocked in September 212 with final harvest expected in December 213. The variable lice abundance, and population structure, recorded within the cages did not reflect the pattern observed within the wild population. In addition, the peak in Caligus recorded in the wild population was not reflected in the cage records. Table 6 The abundance, intensity and median value of the salmon louse on wild sea trout in Laxford Bay, where abundance is the mean number of lice per fish and intensity is the mean number of lice per infected fish. Abundance Intensity Month mean range mean range Median March April May June July August September October Polla The abundance of lice shown in Table 7 was relatively low, with the exception of March which was based on 2 fish. The lice population within the wild fish had a mixed structure throughout the year, with maturation observed between the May and July samples (Fig. 4b). It must be noted, however, that this is unlikely to be the same population as the development times of lice are less that 2 months. Caligus were present throughout the year, with the greatest numbers recorded in May (316). The neighbouring cages were fallow from July. Lice abundance within the sites has remained low with less than.5 Lepeophtheirus per fish (all stages). Caligus densities have been higher. Unlike Lepeophtheirus, Caligus are found on a range of species and densities will vary with the occurrence of marine species such as cod, mackerel and whiting amongst others. Table 7 The abundance, intensity and median value of the salmon louse on wild sea trout in Polla estuary, where abundance is the mean number of lice per fish and intensity is the mean number of lice per infected fish. Abundance Intensity Month mean range mean range Median March April May June July August September October Laxford and Polla lice data, historical analysis As sampling was not consistent throughout the period, data analysis was undertaken on an annual basis. This involved the averaging of lice numbers and prevalence over the year. From Fig. 5 it would appear that there is a pattern in the lice population on the wild fish in both estuaries over time. In general there were a lower number of lice present on wild fish in the first year of production within the neighbouring farm. In the case of the Laxford, the neighbouring farm operates a three year production cycle, with the third year of production, or fallow year, corresponding to a drop in the abundance of lice, the lowest abundances corresponding to the first year of production. This relationship was found to be statistically significant (p <.5). 7

9 No. lice per fish Laxford Polla Year Fig. 5 Showing the abundance of lice per year per estuary This is in agreement with the findings from a variety of other studies (i.e. Butler 22). Additional analysis indicates that there is no statistical difference between fallow years and the first year of production and further supports the fact that the second year of production within the neighbouring farm is likely to have a significant impact on wild sea trout, irrespective of the number of years a system is fallow. However, it should be noted that the 3 year production cycle gives 2 good years to every bad. Within the Polla estuary, there was a change in the abundance of lice observed in 27, with a reduction in the level of fluctuations observed. The reason for this change is not known, although it corresponds with a change in farm operator within the sea loch and the introduction of SLICE treatment within the farm (A. Marsham, pers. comm.). The cages in Eriboll have been through several changes of ownership during the course of this project. Prevalence also shows a statistically significant relationship (p <.5) with year of production, although as with abundance there is no difference between fallow years and the first year of production. Fish within the Polla estuary have a higher prevalence of lice compared to those in Laxford Bay over much of the sampling period (Fig. 6) despite the variations in abundance (Fig. 5). This indicates that the majority of fish within the Polla have lice, but at a low level. A main assumption in many areas is that sea lice infestations are the reason for the collapse in the sea trout populations throughout the west coast of Scotland. In order to assess this, sea trout catches within the neighbouring rivers were compared to the prevalence of lice observed. As it is likely that any resultant mortality will affect the rod catches in the following year, a lag in catches has been introduced (Fig. 7). From this, it can be seen that there is a statistically significant negative relationship within the figures. In order to look at the potential impact of sea lice on the population structure an assessment of the specific growth rates was undertaken (Fig. 8). In order to ensure that interannual variations are removed from the analysis, only fish recaptured in the year of tagging were included. Again there was a significant correlation between both sea lice abundance and prevalence and the specific growth rate of sea trout. However, it must be noted that the prevalence is at the time of recapture, not over the growth period, and therefore the relationship may not be causal. 8

10 8 7 Percent fish with lice Laxford Polla Year Fig. 6 Showing the prevalence of lice per year per estuary No sea trout caught in the following year Prevalence of lice Fig. 7 Showing the relationship between prevalence of lice and the sea trout catch the following year in the neighbouring rivers (ANOVA, p <.1) SGR in year Prevalence of lice Fig. 8 The specific growth rate of sea trout by prevalence of sea lice (ANOVA, p <.1) 9

11 Discussion West Sutherland is on the fringe of aquaculture activity in Scotland, with a low density of sites within the individual sea lochs compared to other parts of Scotland. However, as with other studies, there is a significant relationship between the year of production within the fish farms and the abundance of sea lice on wild sea trout, irrespective of the use of extended fallow periods. While this is in contrast with previous findings in the Laxford estuary (Marshall 23) which found no correlation in lice levels between farmed and wild fish it further demonstrates the need for an extended dataset. It also highlights the need for adequate and sustained lice control within the aquaculture sites. In the presence of a relationship between aquaculture and the presence of sea lice, it is important to determine the likely impact of the increased loads on the wild salmonid populations. From the present study there appears to be a negative effect on both the sea trout population size in the following year, as described by catch statistics, and their growth rate within the year of capture. The two may be related, with smaller fish being less likely to be caught, although there is little evidence for this within the catch records. From the current study it would appear that the aquaculture industry is a contributor to the decline in sea trout populations observed in west Sutherland. This is, however, not the only factor as the decline does not mirror aquaculture production in the area. There is a proven impact of aquaculture on the lice levels within an estuary, further highlighted by the decrease in abundance following the introduction of SLICE treatment in the vicinity of the River Polla, and it is therefore important to maintain adequate control of the lice populations within the farms to ensure a low impact on wild fish populations. From the findings of this and other studies it would also appear to be important to introduce local management targets. Recommendations for further research 1. It is recommended that the current programme be continued in order to maintain the existing dataset. 2. It is recommended that the current programme be expanded to examine other features of sea trout biology in marine areas. 3. It is recommended that further research into the dynamics of the sea trout population in both marine and freshwaters be undertaken. This should also examine the relationship between the resident and migratory components of the population. 4. It is recommended that additional research on the sea lice population be undertaken. References Butler, J.R.A. (22). 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: Marshall, S. (23). Incidence of sea lice infestations on wild sea trout compared to farmed salmon. Bull. Eur. Ass. Fish Pathol. 23(2): WSFT (213). Monitoring of sea trout post-smolts, 212. Unpubl. Report to the West Sutherland Fisheries Trust, Report No. WSFT2/13. Acknowledgements 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 Ross Barnes, Dave Debour, Andrew Marsham and Rex Onions. Thanks also to Reay Forest and Rispond Estates for permitting the work to be undertaken and assisting with sampling. This project has received partial funding from the North & West DSFB and the Scottish Government via RAFTS. DISCLAIMER NOTICE Whilst this report has been prepared by the WSFT biologist on the basis of information that she believes is accurate, any party seeking to implement or otherwise act upon any part or parts of this report are recommended to obtain specialist advice. The WSFT and its biologist do not accept responsibility under any circumstances for the actions or omissions of other parties occasioned by their reading of this report. 1