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 1: Scale reading report

General comments on scale readings

Scale-readings were carried out according to the traditional method [1] . Its basis is to define winter periods of growth as regions of the scale where the rings (circuli) are much closer together. From the number of such winter periods the age of the sea trout can be deduced. Since the distance between circuli reflects the growth rate it is easy to see where first migration to sea occurred – circuli spacing becomes much wider after marine migration because the growth rate is more rapid. In most cases it is possible to infer the numbers of years spent in freshwater before first migration to sea, the number of years spent in the sea and the number of spawning returns made in previous years.

Back calculation to estimate length at earlier winters was performed using the methods described by Nall [1] , where the term ‘winter' always refers to the ‘end of winter' immediately before commencement of the new year's growth. The method is based on measuring the distance from scale centre to any particular winter as a proportion of the distance from centre to edge. This proportion multiplied by the fork length of the fish gives a reasonable estimate of the length of the fish at the end of that winter.

Back calculation provides reasonable estimates in the case of maiden fish, i.e. fish that have not yet spawned, but is not reliable for mature fish which have spawned during previous winter(s). Scales on these fish become eroded at the margins, and new growth if often distorted producing a spawning mark .

Composition of sea trout runs

Scale reading of 574 sea trout captured in river Ouse 1985–2004 has provided a valuable data resource. The sample featured sea trout which weighed from just under 450 grams (1 lb) to 5.67 kg (12.5 lb).

The largest fish tended to be those that had spent the longest period of time in the sea, although this was not always the case. The longest time spent at sea, according to scale readings, was just over 5 years (5+ years), yet the largest fish in the sample (5.67 kg, 12.5 lb) had spent only 2+ years at sea and had one spawning mark only. Table 1 gives numbers of fish according to numbers of years spent in the marine environment since smolt migration.

Sea Years	0+ *	1+	2+	3+	4+	5+	6+	_all
Numbers	6	373	141 †	42	9	3	0	574
% Total	1.0	65.0	24.6	7.3	1.6	0.5	0	100
* The majority of 0+ sea trout (whitling) were below the 38 cm size limit, and were returned without scales being retained. Few whitling are seen before September and most run up after 1 November.
† 34, 2+ sea year were maiden fish without spawning mark; 107 mature with spawning mark.
Table 1 - Numbers of sea years recorded on scales of sea trout captured by anglers 1985–2004

The results suggest that 1+ sea year fish make up around 65% of catches with around 25% of 2+ sea year fish and 7% 3+ sea year fish. The remainder were 0+ sea year fish, i.e. whitling , and older fish >3+ sea years. Around 55% of anglers' catches did not have spawning marks on scales and were presumed maiden fish. The remaining 45% had spawning marks on one or more previous winters which suggested that they were mature . These latter, however, include spawning marks at the whitling stage which, as described below, may be false.

The majority of 1+ sea year sea trout, 74%, appeared to be maiden fish, with 26% showing spawning marks at the whitling stage. These marks were weak in most cases, and it is possible that many resulted from a longish period of residence in freshwater at a time when food was scarce in winter, rather than resulting from actual spawning [1],[2] .

Among 2+ sea year fish, mature fish seem to predominate (75% based on the presence of spawning marks). Spawning marks are generally much better defined than those described for whitling. Around 25% of 2+ sea year fish lacked spawning marks and appeared to have been maiden fish; these were larger on average than those with spawning marks.

Only 1 fish appeared to have 3 maiden sea years before return; all others were mature. All older fish with >3+ sea years were mature fish with two or more spawning marks. It is probable that some of the older fish are 1, or even 2 years older than indicated because erosion at the scale margin which results in a spawning mark can completely remove one or more years growth, including previous spawning mark(s). Such fish will be scale-read as one or more years younger than they actually are. This applies especially to fish with two or more spawning marks.

Survival rates

I have estimated the survival rates of adults , using data from scale readings of rod catches; the results are set out in Table 2.

Sea Years	1+	2+ (mature)	2+ (maiden)	3+ †	4+ †	5+ †
Numbers	373	107	34	41	9	3
est. Survival rate	-	28.7%	-	29.1%	21.4%	-
† All bar 1 sea trout with 3+ sea years were mature; all older fish were mature
Table 2 - Estimated survival rates based on scale readings of anglers' catches

The results suggest that about 29% of 1+ sea year fish return again as mature 2+ sea year fish, and about the same proportion of 2+ sea year fish return again as 3+ sea year fish. The latter might be biased by some older fish being wrongly classified as 3+ due to whole-year erosion resulting from strong spawning marks.

Numbers of older fish appear to fall off very quickly with age but, again, there might be bias. The oldest fish in the sample was a hen fish of (at least) 7+ years total age (freshwater + sea). This fish, which weighed (4.76 kg, 10.5 lb) was taken on 31 May 1999 and had 5+ sea years and 4 clear spawning marks; the scale reading was 2+.1+4SM+.

Further biases might have resulted from:

The anglers not catching a representative number of the larger fish;
Tthe presence of 2+ sea year (or older) fish that had returned on a previous occasion as 1+ sea year (or older) fish during winter, when they could not have figured in anglers' catches.

However, there is no evidence to suggest that the winter fish are much different in size composition than summer and autumn fish – apart from whitling; if anything they appear to be larger, probably because of a longer period of rapid sea growth before returning to the river.

It has not been possible to make estimates of survival rates for pre-smolt stages, but it is clear that the survival rate from egg to smolt must be low, otherwise there would be many more sea trout than there are. The literature [4] suggests that in waters like river Ouse the largest losses occur at the egg and alevin stages, i.e. a smaller proportion of eggs/alevins survive to emerge from redd sites as swim-up fry . These losses at the early stages are partly compensated for by the larger size of the returning adult fish – larger adults produce more eggs per fish, and by the absence of strong density-dependent mortality at the fry and early parr stages due to the low density of young trout.

Growth rates for adult sea trout

The average weights for the main sea year groups identified by scale readings are set out in Table 3

Sea years	1+	2+	3+
Average weights /lb	4	6.5	8.0
Average weights /kg	1.8	2.95	3.6
Weight range /kg	1.0–3.6	1.7–5.7	2.4–5.3
Table 3 - Average weight and range for adult sea trout captured by anglers

These results reveal that river Ouse sea trout have very high average weights for each sea year group, and quite possibly the fastest growing in Britain . According to Nall [1] , who published a fairly comprehensive study of scales from British sea trout in 1930, this variation in size within sea year groups usually accompanies high growth rates in sea trout.

The fastest growing fish reached 4.88 kg (10.75 lb) in just over 3 years – a growth rate hard to achieve in a fish farm! This fish had been a 1+ year smolt in 2002, then after just over 1 year of marine life returned to spawn in late 2003. After spawning the fish returned to the sea early in 2004, before returning for a second time in June 2004. The reading was: 1+.1+1SM+.

A frequent problem in scale-reading river Ouse sea trout is that of indefinite sea winters. Instead of the very closely spaced narrow band depicting the winter, there is only a slight narrowing of spaces between circuli to suggest the presence of a winter. It is possible that fish showing indefinite sea winters on their scales have remained in the marine environment during the winter instead of returning to the river. The phenomena is mostly seen on the scales of maiden 2+ sea year fish, i.e. fish that had remained at sea without returning for just over two years, but is it also seen on the scales of the largest 1+ sea year fish. Trout can feed at low temperatures if there is food available and winter growth checks usually reflect a lack of food rather than hesitation in consuming it due to cold conditions. A visit to any fish farm in winter will confirm this – the fish feed just as enthusiastically as at any other time. I make this point to emphasise that sea ‘winters' on scales may not coincide with the shortest days of winter at the end of the year. Instead, they may chime with periods when food is at its scarcest and this may vary from year-to-year.

Whitling

Whitling are the very small sea trout that run up the river in autumn and winter. They are small because they have spent less than 1 year in the sea, normally 5–10 months, sometimes as little as 3. Usually less than 38 cm in length (15 inches) they do not often figure in anglers' catches, being undersized. Several that were over the size limit were scale-read and were unremarkable except that both had very large smolt sizes. The largest scale-read whitling reported was 43 cm (17 inches). Smaller whitling found dead were in the size range 27–38 cm (11–15 inches). Scale reading of 1+ sea year fish have indicated that autumn whitling should be in the size range 27–44 cm (11–17 inches).

Whitling do not necessarily remain in the lower reaches of rivers during their freshwater residence. Some may move upstream and into the lower sections of tributaries, while others take up residence in tidal reaches for a few weeks or so before returning down the river to the sea. Although scale readings have suggested that around one-quarter of 1+ sea year sea trout had spawned at the whitling stage, winter sea trout watchers' have reported few sea trout of whitling size at spawning sites. However, several cock whitling recovered dead during late winter had kypes and appeared to be kelts ; their scales showed spawning marks forming. So it is clear that at least a few of the whitling do spawn.

Some ‘whitling' appear to be significantly smaller than the figures given above. I have seen several of these small fish, and have had several reports of ‘silvery' trout in the size range 15–25 cm (6–10 inches) from other observers. These may be whitling with 1+ year smolt history or silvery parr . It is not possible to say without direct scale reading, but there were a few instances where a fish with 1+ year smolt history showed evidence of a whitling stage at end of first sea winter.

Sex ratio of returning adults

It is noticeable that fewer cock fish have been captured by anglers than hens; this feature is by no means uncommon in other sea trout rivers [2],[3]. It is sometimes difficult to assign the correct gender to adult sea trout, so I have relied on the results of one angler who assigned gender after internal examinations of the fish he kept [5] . A breakdown of his results from 1985–2004 reveals that there were 112 hens but 49 cocks only, a ratio of 2.3: 1.

The scientific literature [2],[3] recognises that such imbalances usually result from a similar imbalance in smolt runs. In these waters a proportion of the male parr do not migrate – instead they remain as freshwater trout. A common ratio of hen to cock adults in these cases is 3:1. A study of 9 Norwegian rivers [3] revealed that the proportion of male parr not migrating varied between 7% and 56%. The proportion was greater in rivers with fast-growing parr with low mean smolt age , but almost non-existent in rivers with slow parr growth and a larger mean smolt age.

Further studies [3] have shown that such male ‘parr' may become sexually mature without having a marine stage and will mate, or attempt to mate, with large hen sea trout or the smaller hen brown trout in the stream. Such ‘adult' male parr are described as ‘precocious' by analogy with male salmon parr which sometimes exhibit the same behaviour. Of considerable interest is the fact that studies on mature male sea trout parr, i.e. male parr that had already spawned, revealed that they were intolerant to salt water and would not have been able to survive migration to a marine environment.

This behaviour might explain the interesting discovery that in many waters where wild brown trout and sea trout intermingle, the two types are often genetically identical [2] . If separated by a natural impassable barrier of long standing, however, the brown trout were usually found to be genetically different to the sea trout. This suggests that the original strain of brown trout (still found upstream of the barrier) might have been replaced below the barrier by offspring from sea trout ‘reverting' to brown trout behaviour.

If the predominance of females among river Ouse sea trout is matched by the predominance of females in smolt runs, then around one-half of male parr do not migrate! I have no solid evidence that this is the case, nor have I evidence of male parr being observed in the act of spawning. However, on a few occasions when sea trout have been seen in the act of spawning, or just previous to the act, we have seen groups of perhaps 6 or so smaller trout (mostly less than 25 cm, 10 inches) hovering around the vicinity of the redd site. Their presence might suggest motivation to spawn, although they may simply have been there to feed on ‘loose eggs' drifting downstream from the redd site.

It might be thought that with a reduced number of male fish, some females must end up spawning with a parr or larger brown trout. This is probably true but it is important to remember that each male (adult) sea trout will have the capacity to spawn with 2 or 3 females and will fight off smaller brown trout if challenged.

August 2007

References and sources

[1] Nall, G.H. (1930) The Life of the Sea Trout , London : Seeley Service and Co.
[2] 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.
[3] National Rivers Authority (1992) Fisheries Technical Report 3 – Sea Trout Literature Review and Bibliography (and references cited therein), (compiled by Institute of Freshwater Ecology , Windermere), Bristol : HMSO.
[4] National Rivers Authority (1992) Fisheries Technical Report 1 – Sea Trout in England and Wales (and references cited therein), Bristol : HMSO.
[5] Long-standing member of Ouse Angling Preservation Society (1984–2004), Private communications.

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