Monitoring of sea trout post-smolts, 2011

Transcription

1 Monitoring of sea trout post-smolts, 2011 A report to the West Sutherland Fisheries Trust, Report No. WSFT2/12 January 2012 Shona Marshall Fisheries Biologist West Sutherland Fisheries Trust Gardeners Cottage Scourie By Lairg Sutherland IV27 4SX

2 Monitoring of sea trout post-smolts, 2011 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 2003). 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 to, at low tide. Sampling was performed using a 50 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 re-captures, 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 was sampled twice during as part of the RAFTS Aquaculture Project ( 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 impact (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 = 100W/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))*100)/time), where weight is in grams and time in days. Results and Discussion The largest catch within a single sweep was 286 fish in the Laxford estuary during April (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. The exception to this was a large number of mature sandeels in the Laxford in August. August also saw large numbers of cuddies in both estuaries. 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 April May a a July August ( released; a two sweeps during the month) 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 three estuaries was similar, with the Polla returning the oldest adult (Fig. 1). From Fig. 1 the predominant smolt age in all rivers is 2 years (S2), although there were a number of S3 s also present. S1 s were also observed in small numbers in all of the estuaries. The length distribution of fish in each estuary was similar (Fig. 2). 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 April. A May smolt run is normal for the Sutherland area (WSFT 2011). While this appears to be earlier in 2011 it is possible that this is a function of sampling time. Table 2 The percentage of smolts within the catch Month Laxford Polla estuary Bay 0 - April 65 - May July August 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 of the tagged fish recaptured during 2011 were taken in the same location as originally tagged. This confirms findings from previous years (WSFT 2011). 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 appears to increase slightly with time in both estuaries. However, the condition index in the Laxford remains lower than that in the Polla throughout the year. This is despite the presence of a greater number of bait species within the Laxford sweeps compared to those in the Polla. Length appears to vary with time, decreasing over the sampling period before increasing towards the end. 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 60 50 Laxford Polla Kyle of Durness 40 No. fish Age Fig. 1 The number of fish of each age taken in the estuaries No. fish Length (to lower cm) Fig. 2 The number of fish of each length taken in the estuaries 3

5 Table 3a The mean length, weight, and condition index of the post-smolts captured in Laxford Bay, per month Month Mean length (± s.d.) (mm) Mean weight (± s.d.) (g) Mean Condition Index (± s.d.) April ± ± ± 0.13 May ± ± ± 0.27 July ± ± ± 0.00 August ± ± ± Table 3b The mean length, weight, and condition index of the post-smolts captured in the Polla estuary, per month Month Mean length (± s.d.) (mm) Mean weight (± s.d.) (g) Mean Condition Index (± s.d.) April May ± ± ± 0.09 July ± ± ± 0.10 August ± ± ± Recaptures There were 13 recaptures during 2011, all within the estuary netting. The growth of recaptured trout is shown in Table 4a for the Polla estuary and Table 4b for Laxford Bay. Of the recaptured trout, 4 were originally tagged in 2009, the rest in This gives yet more information on sustained growth rates and demonstrates the potential effectiveness of the tagging programme. All 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 7.73 mm, and 4.71 g within the Laxford, which is a slightly greater length increase but lower weight increase than that found in Within the Polla average growth rates were 8.07 mm and g, which show a significant increase in both length and weight growth compared to Table 4a The lengths and weights of recaptured trout within the Polla estuary Tag number Tagged Recaptured Difference Date mths C79 Length (mm) Weight (g) Date mths D27 Length (mm) Weight (g) Date mths I67 Length (mm) Weight (g) Date mths I82 Length (mm) Weight (g)

6 Table 4b The lengths and weights of recaptured trout within Laxford Bay Tag number Tagged Recaptured Difference Date mths D61 Length (mm) Weight (g) Date mths H31 Length (mm) Weight (g) Date mths C18 Length (mm) Weight (g) Date mths D68 Length (mm) Weight (g) Date mths D05 Length (mm) Weight (g) Date mths K96 Length (mm) Weight (g) Date mths C30 Length (mm) Weight (g) Date mths C01 Length (mm) Weight (g) Date mths H68 Length (mm) Weight (g) Figure 3 shows that the specific growth rates in the Laxford and Polla remain low compared to previous years. However, the greatest concern remains in the Laxford, where growth rates continue to decline. While the growth rate in both estuaries remains low, being the lowest ever recorded in the Laxford, by catch within the sweeps would indicate that there is prey available. The particularly poor condition of the fish in 2010, described by Fish Health as anorexic (WSFT 2011), will have had some impact in the 2011 results, especially in the Laxford where most recaptures were taken in April, however this can only be part of the problem. The results demonstrate the complexity of trout population dynamics and the interactions with external factors, such as food supply and temperature. Average SGR Year Laxford Polla Fig. 3 Showing the average SGR for fish within the Laxford and Polla estuaries, by year 5

7 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. While a mixture of lice stages were noted throughout the sampling period in the Polla, the Laxford contained only Chalimus stages, with the exception of the sample when a small number of adults and mobiles were present (Fig. 4). No gravid females were present within the Laxford samples. Lice numbers were variable within the sampling, the greatest number being found in the Laxford in. Total lice number per sample is, however, dependent on sample size and the use of Abundance and Intensity data gives 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 67 - April 7 - May July 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 infestations, 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 (2002), 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 from a single heavily infected individual. Laxford Lice abundance within the Laxford samples remained variable throughout the year, with lice numbers declining with time (Table 6). While high abundances such as this have been observed previously, it has been on one occasion within the year rather than a constantly high level of abundance. All samples were dominated by juvenile lice, with little maturation of lice observed (Fig. 4a). This pattern has not been seen previously within the Laxford and would suggest the presence of a constant source of lice out with the sea trout population. 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 April May July August Polla As with the Laxford, the abundance of lice shown in Table 7 is variable throughout the year, increasing to July and then decreasing. In each month the full range of life stages was observed from juvenile to gravid (Fig. 4b). This indicates a mature lice population, a situation frequently observed within the Polla, suggesting that the lice population is being maintained within the wild fish population in Loch Eriboll. While lice abundance, and the median value, is higher than that seen in recent years it remains significantly lower than that seen in the Laxford. 6

8 The neighbouring cages were fallow from the start of July. In the period prior to fallow there were no juvenile stages recorded, while numbers of mobile and adult stages varied but remained significantly lower than those found on the wild. 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 April May July August 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 This data series is extensive and has been analysed to test a number of relationships between the wild and farmed fish. In particular, the stage of production within the farm is considered by many, including the aquaculture industry, to be important to the abundance of lice on wild fish. Prevalence of lice within the wild fish population was the first relationship assessed. Figure 5 shows the prevalence of lice with time in the two estuaries. From this it can be seen that there is an annual trend present, with prevalence increasing to and then declining. From this it would therefore appear that there is no relationship between prevalence and stage of production. However, statistical analysis of the combined annual data revealed that there was a highly significant relationship present. By showing the differences in abundance with time, trends in the data can be assessed. There is evidence from many areas that the abundance of lice on wild fish is related to the year of production on the neighbouring fish farms, being higher in the second than the first year. This pattern is also widely accepted within the farms themselves, and this analysis enables the trend to be assessed in the Sutherland area. Changes in the abundance of lice on the wild fish (Fig. 6) appear to follow a recognisable pattern, with higher abundance apparent in the 2 nd year of production within both the Laxford and Polla samples. By showing the same data as a 2 period moving average (Fig. 7) the trends within the data are more apparent. From the graphs there does appear to be a pattern, and statistical analysis reveals this to be significant. However, this pattern does appear to break down over the past 2 years. Catch returns for rod caught sea trout in the two rivers are shown (Fig. 8). From this it can be seen that the catches in the Laxford showed an increase during the early 2000 s, before declining after In contrast, the Polla catches increased at the start of the century before declining prior to a slight increase and levelling out. Statistical analysis of these data indicates that there is a significant relationship between the sea trout catches and the stage of production on the neighbouring farm. 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. 5. It is recommended that the poor condition and growth rates observed are investigated. 7

9 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: Marshall, S. (2003). Incidence of sea lice infestations on wild sea trout compared to farmed salmon. Bull. Eur. Ass. Fish Pathol. 23(2): WSFT (2011). Monitoring of sea trout post-smolts, Unpubl. Report to the West Sutherland Fisheries Trust, Report No. WSFT2/11. 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 the proprietors and staff of Reay Forest and Rispond Estates. 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. 8

10 Chalimus Mobile Adult Gravid females % lice April May July August 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 April May July August 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. 9

11 month Fig. 5 Showing the changes in prevalence of lice with time for the different estuaries Laxford Polla % with lice 10

12 11

13 Laxford Polla 2 per. Mov. Avg. (Laxford) 2 per. Mov. Avg. (Polla) time Fig. 7 Showing the changes in abundance of lice with time in each estuary lice a bundance 12

14 No. fish Laxford Polla Year Fig. 8 Showing the sea trout catch per year for the two rivers 13