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

Introduction

Atlantic salmon ( Salmo salar L.) , are fish of great cultural importance, which for many years have supported substantial fisheries in countries bordering the North Atlantic Ocean. In most regions salmon live in fresh water for 1-4 years before smolting and migrating to sea. The fish then grow rapidly at sea before returning after one winter, as grilse (1SW), or longer, as multi-sea-winter ( MSW) salmon. The species is currently in decline in many parts of its range, probably due in large part to reduced survival at sea ( ICES, 2014). This pattern of decline has focussed attention on the need to maximise the output of high quality smolts from rivers to mitigate, where possible, reductions in numbers of returning adults.

The numbers of smolts produced in river systems are limited by the availability of suitable habitat, which varies with fish size, and the presence of competitors and predators (Armstrong and Nislow, 2006). Stocking of early life stages is therefore likely to invoke competition with wild-reared fish (Metcalfe et al., 2003). A potential option for increasing production of smolts beyond the capacity of the river is to rear salmon in aquaculture facilities and stock them into the river only when they are ready to migrate to sea. This process is termed "ranching". Salmon are believed to imprint on specific odour cues during their out-migration and to use these to locate home reaches on their return to fresh water ( e.g. Hasler and Sholz, 1983). Hence, ranched fish are stocked in rivers above the site to which they are required to return.

Although the technique of ranching salmon has been advocated by some fishery managers in Scotland, there appears to have been little formal analysis of the method in the Scottish context. Such information is important for evaluating the economies of potential fisheries based on ranching, and for assessing aspects of the value of the method for conservation purposes. Although there has been some review of a range of studies of survival of either wild or hatchery-reared smolts in the UK and Ireland (Aprahamian et al., 2003), only the studies of marine survival in the River Bush (Crozier and Kennedy, 1993) and the Burrishoole system (Piggins and Mills, 1985) have directly compared the two categories of fish during the same years within the same river system.

To investigate the effectiveness of smolt ranching as a tool to enhance local salmon fisheries, we examine first, the range of recapture rates from monitored smolt releases in Scottish rivers and second, we compare returns from hatchery and wild-reared smolts originating from the same stocks released concurrently at the same river sites. We then consider a range of practical issues associated with operating a smolt ranching scheme on a Scottish river, both for conservation and fishery enhancement purposes, and then, using best available information, assess the likely financial implications.