Sea Trout Scale Reading Report

About Sussex Ouse Sea Trout

Sussex Ouse Sea Trout Studies

Part 1 - Scale reading report

Part 2 - Smolt age

Part 3 - Effects of dry & wet years on survival

Sea Trout Glossary

Part 3: Effects of a dry year (1996) compared with a normal-wet year (1999) on survival and growth of sea trout in their 1st parr year

General comments

1996 was a very dry year. The previous year had also been dry, and rainfall was well below average for the winter 1995/6. The dry period continued throughout the spring and summer of 1996; the only respite was above-average rainfall in August, but September and October were again very dry months. By contrast 1999 was an average to wet year with good rainfall the previous winter, and above average rainfall during summer months with the exception of July . Monthly rainfall totals [3] for both 1996 and 1999 are shown in Figure 1.

Figure 1 - Monthly rainfall totals: top 1995/6; bottom 1998/9

Catches in 1996 and 1999

Comparison of catches during the period shows that both 1999 and 2002 were very good years for sea trout fishing (Table 2). Sea trout having their 1 st parr year in 1996 returned as 1 sea year fish with 2-year smolt history in 1999, while those with 1 st parr year in 1999 returned as 1 sea year fish with 2-year smolt history in 2002.

Year	1995	1996	1997	1998	1999	2000	2001	2002	2003	2004	2005	2006
Yearly catch	41	9	33	38	133	78	146	161	101	76	27	41
† Source: Environment Agency web site. Copyright © Environment Agency
Table 2 Anglers' sea trout returns for river Ouse ( Sussex )†

In both years sea trout catches there were high proportions of 1 sea year fish (1999, 83%; 2002, 81%) the majority of which had 2-year smolt history. These results suggest that a dry year with low flows and high temperatures does not necessarily lead to a smaller return of 1 sea year fish, three years later, due to higher than normal mortality rates among 1 st year parr. The implication is that survival rates for river Ouse sea trout in their 1 st parr year are not adversely affected by low-flow conditions, even if these persist throughout the summer.

Parr growth in 1996 and 1999

There were, however, marked differences in 1 st year parr growth between the two years 1996 and 1999. In 1996, 1 st year parr growth was less than that for 1999, the mean size at 1 st parr winter being smaller by 0.6 cm. There were also differences in the proportion subsequently migrating as 2A or 2B smolts (see definitions in part 2, and Table 3 below).

Smolt History	2A	2B	% 2A	Mean length at 1st parr winter
1999 catch	11	22	33%	7.8 cm
2002 catch	28	20	58%	8.4 cm
Table 3 Smolt history in 1999 and 2002 catches

Bar charts illustrating these results are shown below as Figures 2 and 3.

Figure 2 Calculated lengths at 1 st parr winter of sea trout with 2.1+, 2+.1+, 2.0+1SM+ and 2+.0+1SM + histories captured in 1999. Growth year: 1996

Figure 3 Calculated lengths at 1 st parr winter of sea trout with 2.1+ and 2+.1+ history captured in 2002. Growth year: 1999

The results do show, however, a few fish achieving very high 1 st year parr growth (>12 cm) in both years. It is tempting to suggest that these were in waters less affected by the drought in 1996, but it is a fact that trout growth is very variable even when food is readily available, such as in a fish farm. By the same token, there are a few fish with quite low 1 st year parr growth in 2002 catches – despite 1999 being a wet year.

The results for 1999 catch include those from 1 sea year fish with weak spawning marks at whitling stage. These are not usually measured because the spawning mark may result in some of the previous growth being eroded and so back calculations will not be accurate. Those with weak spawning marks, however, are not certain to have spawned and there is evidence that such spawning marks can be misleading [4] ; a weak spawning mark might result from a whitling return followed by a period of winter residence in freshwater without spawning. However, if allowance is made for erosion (usually less than an amount representing 2 cm growth along the axis of measurement) the error can be seen to be small if considering the short 1 st parr year growth.

For example if the fish had measured 50 cm and the calculated length at 1 st parr winter was 10 cm, then the ‘real' length allowing for 2 cm erosion would be:

50 x 10/(50 + 2) = 500/52 = 9.6 cm.

So results from mature fish with slight erosion due to weak spawning marks may be slightly too high, but within the limits of experimental error (these usually ±5%).

The reason for using these extra results was to increase the sample size from 1999 catch to above 30. In general, you need at least 30 results before you can calculate a reasonable average length. Less than 30 results give a less reliable average and usually a less convincing bar chart of results. Furthermore, the results must be a random selection from the population being studied; i.e. not selected for size or by month of year when captured.

Similar results were determined for 1 st year parr growth in very dry year 1990; in this case, scale readings from 1993 catch were examined. The majority has 2B-smolt history, and the average length at 1 st parr winter was 7.8 cm. However, the sample size was only 15 maiden fish with 2-year smolt history so not statistically sound. However, 1993 was a good year for sea trout catches compared with previous years.

Conclusion

The results from 1999 catch, which were used to examine growth in 1996, suggested that survival was surprisingly good during the warm dry year 1996, there being no significant reduction in numbers of 3+ aged fish in 1999 catch compared with 2002.

The results did show, however, that 1 st year parr growth was significantly reduced in 1996 compared with growth in 1999. Moreover fish with 2B smolt history, which were the majority in 1999 catches, were a minority in 2002 catches, while fish with 2A smolt history, which predominated in 2002 catches, were a minority in 1999 catches.

The results strengthen the view that length at 1 st parr winter strongly influences whether the parr subsequently becomes a 2A smolt or 2B smolt.

January 2008

References and sources

[1] Le Cren, E. D. (Ed) (1985) The Biology of the Sea Trout (and references cited therein), Summary of a Symposium held at Plas Menai during 24–26 Oct. 1984: Atlantic Salmon Trust.
[2] NRA (1992) Fisheries Technical Report 3 – Sea Trout Literature Review and Bibliography (and references cited therein), (compiled by Institute of Freshwater Ecology , Windermere), Bristol : HMSO.
[3] All rainfall data courtesy of Robin Pepper, Sussex Ouse Conservation Society.
[4] The Sea Trout of River Ouse , Sussex : Part 1, Sussex Ouse Conservation Society,

Copyright © (2008) Clive L. Fetter. Reproduced by kind permission of the author.