Stress and survival of small pike-perch Sander lucioperca (L.) after trawling and chilling P. HYVA ¨ RINEN*†, V. LEPPA ¨ NIEMI ‡, K. J OHANSSON‡, P. KORHONEN* AND P. S UURONEN* *Finnish Game and Fisheries Research Institute, Kainuu Fisheries Research, Manamansalontie 90, FIN-88300 Paltamo, Finland and ‡Department of Biological and Environmental Sciences, Faculty of Biosciences, Viikinkaari 9, P. O. Box 56, FI-00014 University of Helsinki, Finland (Received 10 May 2006, Accepted 3 March 2008) The mortality and stress responses (plasma cortisol concentration) of undersized (total length, L T , <370 mm) pike-perch Sander lucioperca caught as by-catch in a trawl fishery were assessed. The effects of three different holding methods on pike-perch caught by trawl were compared: (1) fish that were transferred to ambient-temperature water (150–214° C) immediately after capture and not exposed to chilling water, (2) fish held in chilling water for 10 min and (3) fish exposed to chilling water for 2 h. The sample fish were held and monitored in individual plastic restrainers at ambient-water temperature for 48 h after each of the handling procedures. Total mortality was similar for fish held in chilling tanks for 10 min (272% total mortality) compared to fish that were only held in ambient-temperature water (282% mortality). Total mortality reached 913% when fish were held in chilled water for 2 h. Mortality and plasma cortisol concentrations correlated inversely with size (96–368 mm L T ) and directly with lake-water temperature (150–214° C) after treatment. In addition, the effect of chilling was observed in elevated cortisol concentration and delayed start of recovery. The results show that quickly removing (<10 min) undersized pike-perch from chilled water could markedly improve the survival of released fish. Due to delayed recovery, however, fish should be allowed to recover (30–40 min) in a deck tank before release. # 2008 The Authors Journal compilation # 2008 The Fisheries Society of the British Isles Key words: by-catch; chilling; mortality; pike-perch; Sander lucioperca; trawling. INTRODUCTION Several studies have shown that trawling causes stress, injury and mortality to fishes that are returned to the water (i.e. discarded). It has also been demon- strated that trawled and subsequently released fishes can recover from the capture and release process and survive (Turunen et al., 1994; Jurvelius et al., 2000; Davis & Olla, 2001; Davis et al., 2001). Survival rate, however, †Author to whom correspondence should be addressed. Tel.: þ358 205751641; fax: þ358 205751649; email: pekka.hyvarinen@rktl.fi Journal of Fish Biology (2008) 72, 2677–2688 doi:10.1111/j.1095-8649.2008.01885.x, available online at http://www.blackwell-synergy.com 2677 # 2008 The Authors Journal compilation # 2008 The Fisheries Society of the British Isles