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 2: Smolt age

General comments on early freshwater growth of River Ouse sea trout

Early attempts to determine smolt ages by scale readings from returning adults suggested a mixture of smolt ages from 1 to 4 years, with 2-year and 3-year smolts predominating. Back calculations, however, suggested that in many cases the 3 rd freshwater ‘year' showing on sea trout scales with ‘3-year' smolt history was in fact spring growth preceding the transformation to smolt, rather than a whole year's growth. Scale readings of small brown trout captured in April / May also showed a similar spring growth. This spring growth was also showing on most sea trout with 1-year smolt history (2 nd spring growth) and on several fish with 3-year smolt history (4 th spring growth). Smolts with extra 3 rd spring growth have been reported from other waters [1],[2] , generally where spring comes early. River Ouse smolts have been encountered from the end of winter until late spring: anglers have returned smolts as early as mid-January; Environment Agency smolt trapping has revealed smolt runs as late as mid-May.

Smolt age and growth variants

Table 1 gives a summary of all of the smolt types found in the large 1985–2006 sample, giving smolt age and growth characteristics. Smolts described as type ‘A' have scale readings consistent with migration at the end of winter, while those described as type ‘B' are consistent with migration during late-spring. Consequently, ‘B' types show a short period of parr growth following the last winter before smolt migration. The amount of spring growth revealed by scale reading suggests that there are peaks of migration at both ends of the period late winter through to late spring, rather than a steady trickle of smolts throughout the period. It would be useful to be able to read scales from both winter and spring smolts to confirm that ‘3 rd spring growth' really does relate to spring migration, as the sea trout literature [1],[2] has shown to be the case in other rivers, and its absence to late winter migration.

Sea trout with 3-year smolt history usually give clear readings, although a few seem to have growth intermediate between 2B- and 3-year smolt history types. Those with 4-year smolt history are often more problematic. Some of the latter might have younger smolt history with slow growth and summer growth checks during parr life, or possibly with 3+ smolt history. In any event, numbers with 4-year smolt history are low.

Because proportions with smolt history other than 2 years are low, these are likely to be absent from catches of less than around 15 fish. This is often the case, with odd ones popping up at random as you might expect, but there have been clusters of fish with uncommon smolt ages in particular years. These may not be significant because samples (or sub-samples) need to consist of scales from 30 or more adult fish in order to be statistically credible. This constraint means that further analyses of scale readings to sea trout other that those with 2-year smolt history will need much larger samples, and this is best accomplished by directly reading the scales of actual smolts, rather than those from adult fish.

Smolt type †	smolt description: age and growth	percent of catch (approx.)
1B	1+ year smolt with 2 nd parr spring growth • variant has unusually rapid growth during 2 nd parr spring	9
2A	2 year smolt without 3 rd parr spring growth • variant has unusually rapid growth during 2 nd parr year	39
2B	2+ year smolt with 3 rd parr spring growth • variant has unusually rapid growth during 3 rd parr spring • variant has unusually rapid growth during 2 nd parr year and 3 rd parr spring (rare)	38
3A	3 year smolt without 4 th parr spring growth	10
3B	3+ year smolt with 4 th parr spring growth (very rare)	-
4A	4 year smolt without fifth parr spring growth	4
† See references [1],[2]
Table 1 - Smolt ages and growth variants

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.

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