The relationship between stocking eggs in boreal spawning rivers and the abundance of brown trout parr

ICES Journal of Marine Science ICES Journal of Marine Science (2015), 72(5), 1389– 1398. doi:10.1093/icesjms/fsv017 Original Article The relationship between stocking eggs in boreal spawning rivers and the abundance of brown trout parr Jukka Tapani Syrjänen 1 *, Timo Juhani Ruokonen 1, Tarmo Ketola 2, and Pentti Valkeajärvi 3 Department of Biological and Environmental Science, University of Jyväskylä, PO Box 35, Jyväskylä FI-40014, Finland Centre of Excellence in Biological Interactions, Department of Biological and Environmental Science, University of Jyväskylä, PO Box 35, Jyväskylä FI-40014, Finland 3 Finnish Game and Fisheries Research Institute, Survontie 9, Jyväskylä FI-40500, Finland 2 *Corresponding author: tel: +358 505454615; fax: +358 142602321; e-mail: jukka.t.syrjanen@jyu.fi Syrjänen, J. T., Ruokonen, T. J., Ketola, T., and Valkeajärvi, P. The relationship between stocking eggs in boreal spawning rivers and the abundance of brown trout parr. – ICES Journal of Marine Science, 72: 1389 –1398. Received 27 August 2014; revised 22 December 2014; accepted 14 January 2015; advance access publication 10 February 2015. Stocking with eggs has been widely used as a management measure to support degraded salmonid stocks. In Finland, Atlantic salmon and both seamigrating and lake-migrating brown trout are stocked as eggs, alevins, fry, parr, and smolt, whereas trout are also stocked as mature fish. The aim of this stocking is to improve catches and to support collapsed spawning stocks. We assessed the success of stocking with brown trout eggs in a study of 17 Finnish boreal forest rivers, of which 9 were subject to egg stocking. All rivers contained some naturally spawning trout. In 16 rivers, including nonstocking years and unstocked rivers, egg stocking did not increase the total (wild and stocked) density of 0-year-old parr. However, those rivers with higher existing trout densities in non-stocking years seemed to benefit most from stocking, suggesting some role of river-specific extrinsic factors affecting egg-to-parr survival. In one river monitored for 14 years, only a weak correlation was found between the total density of 0-year-old parr and the number of eggs stocked. However, in nine parr samples from five rivers, the mean proportion of parr derived from stocked eggs was 40%. The mean survival to first autumn parr of egg-stocked and wild individuals was 1.0 and 3.3%, respectively. Probable reasons for the detected low to moderate impact of egg-stocking are (i) large variation in total parr density between years and rivers, (ii) small number of stocked eggs, (iii) placing egg boxes and egg pockets in unsuitable microhabitats, and (iv) unsuitable emergence time of egg-stocked individuals, or other extrinsic factors creating extra mortality. We recommend field and laboratory experiments to improve and standardize stocking methods, and monitoring the connection of wild spawning stocks and parr recruitment. Finally, we encourage fishery authorities to create clear management goals for threatened wild salmonid stocks. Keywords: alevin, Alizarin red, egg box, egg pocket, otolith, redd, Salmo trutta, stock management, survival. Introduction Stocking of eggs or hatched alevins has been widely used as a management measure to support natural parr production of salmonids in rivers (Prignon et al., 1999) and lakes (Bronte et al., 2002) or to expand their natural distribution range. Various methods have been used, including eggs in pipes that are pushed into gravel, egg boxes, and pouring eggs directly onto the bottom substratum (Barlaup and Moen, 2001; Kirkland, 2012). Eggs or alevins, rather than parr, have been used in stocking, as eggs are cheaper to stock than parr and are easy to transport (Johnson, 2004). Moreover, eggstocked fish go through most phases of their lifespan in their natural environment, and thus experience natural selection, which could keep the genotypes and phenotypes of populations as near to natural as possible (Kirkland, 2012). Globally, egg stocking has been used for stocks of the genus Salmo for more than a century (Kirkland, 2012). The success of the action could be estimated as egg-to-parr survival or as comparisons between different stocking methods. However, most studies are cases of one river, one year, and one method (Beall et al., 1994; Raddum and Fjellheim, 1995; Coghlan and Ringler, 2004) or are limited only to the alevin phase (Kirkland, 2012). Barlaup and Moen (2001) reviewed egg stocking or egg incubation methods, but could report egg/alevin survival only until alevin hatching and emergence. # International Council for the Exploration of the Sea 2015. All rights reserved. For Permissions, please email: 1 1390 Material and methods Hatchery stocks The Finnish Game and Fisheries Research Institute (FGFRI) maintains 12 brown trout hatchery stocks from Finnish inland waters. In the Finnish Lake District, there are two stocks: the Rautalampi stock from the Kymijoki watershed and the Vuoksi stock from the Vuoksi watershed (Figure 1). The Rautalampi hatchery stock was renewed by few migratory mature fish in the 1990s from the Lake Päijänne region, midst of the study area, but after that, no wild mature lakemigrating spawners have been caught in the rivers. First hatchery generation used in egg production at the beginning of the 2000s were grown from the gametes of these wild fish. In the 2000s, the hatchery stock has been renewed by wild parr sampled in the rivers of the Rautalampi watercourse, such as Taikinainen and Karinkoski which are included in this study as stocked rivers. Parr were grown to maturity in the hatchery. These originally wild fish were also used as spawners in the hatchery, but their eggs comprised 10% of eggs used in this work, and 90% of eggs were from the first or second hatchery generation. During 2006–2011, the Rautalampi Figure 1. Location of the Kokemäenjoki (Ko), Kymijoki (K), and Vuoksi (V) watersheds in southern Finland. The location of the Simunankoski Rapids is shown with latitude and longitude values. Lakes are shown in grey. stock has produced 40 –60 l of eggs annually, 1 l containing 6555 eggs as an average between years (SD 482) (R. Kannel, FGFRI, pers. comm.), that have been stocked into some tens of rivers of the Kymijoki and the Kokemäenjoki watersheds (Figure 1). Since 2008, trout eggs produced by the FGFRI have been marked by adding Alizarin red S to the egg tanks to create colour marks in alevin bones. Alizarin can then be detected in the fish otoliths in the laboratory, although the fish must be killed for this. The otoliths are analysed under ultraviolet light with a fluorescent microscope, and a clear fluorescent area is seen in the centre of the otolith of an Alizarin-coloured fish. Study rivers The study area covers all the most important watercourses and approximately half of the most important free spawning rivers for brown trout in the Finnish Lake District. Seventee (...truncated)