Scottish Marine and Freshwater Science Vol 6 No 5: Ranching to the rod: an evaluation of adult returns from hatchery-reared Atlantic salmon smolts released in Scottish rivers

Discussion

The study revealed that rod capture rates of hatchery-reared salmon smolts returning to Scottish rivers ranged between 0 and 0.23% of fish stocked, with the majority (87.5%) of recaptures caught as 1 SW fish. Capture rates are the product of number of returning fish and exploitation rate. On the River Spey (Scotland), recapture rates were 2-25%, with the highest rates occurring in fish tagged in spring and lowest rates in those tagged in summer and autumn (Thorley et al, 2007). It appears that in general, MSW salmon are more vulnerable to capture by angling than 1 SW fish (Gee and Milner 1980; Erkinaro et al. 1999; Thorley at al. 2007). Rod exploitation rates are known to vary among regions, river catchments, and component stocks. For example, on Icelandic rivers exploitation rates may be 36-85% (Johannsson et al. 1996). On the Tana River (Norway) Erkinaro et al. (1999) report rates of 29% for 1 SW salmon and 51% for MSW fish. By contrast, more southerly rivers appear to have exploitation rates more similar to those on the Spey. On the River Bush (Ireland), Crozier and Kennedy (2001) found that rod exploitation rates of mainly 1 SW fish were 10.9% for wild fish and 11.1% for ranched salmon. Whereas on the River Burrishole (Ireland) exploitation of ranched fish, again predominantly 1 SW, was 10.8% (Mills and Piggins, 1983).

The recapture rates of rod-caught fish from CWT tagged smolts reported must be considered minimum values, as it was not possible to account for any non-reporting of tags or tag loss that may have occurred. However, this is not a consideration concerning the PIT tagging experiments on the River Conon in which returning fish were detected automatically. Accounting for expected angler exploitation rates, the low returns of hatchery-reared fish on the Conon are of the same order as those from coded wire tagging experiments.

In comparative tests the magnitude of difference in favour of wild-reared smolts found on Scottish rivers was approximately 10-fold and therefore of similar order but in some cases higher than in other regions. For example, 3-fold on the River Imsa (Norway) (Jonsson et al. 2003) and 4.5-fold on the Simojoki river (northern Finland) (Saloniemi et al., 2004). In Ireland, survival of wild compared to hatchery-reared smolts on the River Bush (Crozier and Kennedy, 1993) was 4 to 8-fold, whereas on the Burrishoole system, return rates of wild smolts were on average 5-fold greater (Piggins and Mills, 1985). On Iceland's Ellioaár River, Isaksson (1979) reports survival of microtagged wild smolts approximately 7-fold higher than directly planted hatchery-reared smolts.

It is well established (Einum and Fleming, 2001; Weber and Fausch, 2003; Jonsson and Jonsson, 2006) that artificial rearing of salmonid fishes affects their subsequent biology. In captivity they are held at high densities relative to wild-reared fish, feed primarily on artificial food and may lack the abilities to recognise and catch wild prey effectively. Compared with wild-reared fish, hatchery-reared salmon are a different shape, have lower swimming capabilities, reduced social awareness, and poorer predator recognition and escape behaviours. Indeed, the overall brain mass is smaller in hatchery than in wild-reared salmonids, probably reflecting the simpler environment in which they grow (Lema et al ., 2005; Kihslinger et al ., 2006). It appears that the longer fish are kept in artificial rearing facilities before they are released, the less well adapted they are to survival in the wild (Milot et al. 2013; Young, 2013). Therefore, poor survival of hatchery-reared smolts is to be expected. Despite extensive periods of stocking on the Rivers Tyne and Thames (England) with both hatchery-reared parr and smolts as part of a rehabilitation programme, both Milner et al. (2004) and Griffiths et al. (2011) concluded that natural processes (recolonisation, improved water quality, better access) were more important to recovery than initial pump-priming with hatchery-reared fish. Similarly, on the River Sundalslagen, western Norway, extensive stocking with hatchery-reared parr and smolts over a ten year period was estimated to have produced between 0.003 (one for c. 33,000 stocked fish) and 0.028% (one for c. 3,570 stocked fish) rod recaptures to the river, and it was concluded that in most years the number of wild fish taken for brood stock exceeded the number of adult fish recaptured from their offspring (Saltveit, 2006).

From a fisheries enhancement perspective, any hatchery programme is usually required to be economically viable. The production of approximately 250 hatchery-reared S1s was required to generate each returning adult to the River Conon. At an assumed exploitation rate by the rods of 10% (similar to rates found in Irish rivers, Mills and Piggins, 1983; Crozier and Kennedy, 2001, and within the range found on the nearby River Spey, Thorley et al ., 2007), it would have required the production of 2500 smolts for each rod-caught adult. At an estimated £1.50 smolt-1 (S. McKelvey, unpublished data) the cost of each rod-caught fish would have been £3750. This cost needs to be weighed against the economic benefits brought to the fishery which, according to Aprahamian et al . (2003), can be calculated from the additional number of fish caught multiplied by their capital value. Using a capital value for 1995 of £5925 (Postle and Moore, 1996), it is evident that the proposed capital gain of each ranched fish exceeds the costs of producing it, even before adjusting for inflation. However, it is questionable whether such an approach is robust because the value of the fishery might be maintained only by continual, annual investment. Hence, any capital gain would be outweighed by smolt rearing costs after only two years. An alternative approach is to consider the benefit to households from fishing revenue. Butler et al . (2009) calculated that each salmon caught on the River Spey in 2003 was worth £970 to annual household incomes. Using this approach, the annual benefit of each hatchery-reared rod-caught salmon (£970) is far less than the cost of producing it (£3750).

In reality, such simple calculations are unlikely to capture the essence of the values of rearing fish to the economies that they support. For example, there is no account taken of season caught, method of capture or size of fish. All of which may act to influence value. Also, it is unlikely that there is a simple linear relationship between rod catches and expenditure by anglers. In some cases, the fact that fish are being stocked may itself stimulate increased expenditure associated with a fishery, regardless of the actual survival of the fish. However, an understanding of the survival and production costs of hatchery-reared smolts is useful for enabling well-founded decisions on allocation of resources to options for restoring, maintaining and enhancing fisheries.

If ranching salmon is to be employed as a tool for enhancing fisheries in Scotland, then it is probably important to consider what measures can be taken to enhance smolt quality and hence marine survival, and at what additional cost. There are further considerations beyond finance. For example, there is a potential for hatchery-rearing to increase levels of unfit genes in wild populations (Araki et al. 2009; McGinnity et al., 2009; Christie et al. 2012). There may also be potential for large numbers of stocked smolts to attract predators that then deplete numbers of wild-reared salmon. However, the general extent and likelihood of such hazards remains to be evaluated.