Predators Are Attracted to Acclimation Cages Used for Winter Flounder Stock Enhancement

Abstract

Acclimation cages are used for juvenile, cultured winter flounder Pseudopleuronectes americanus so that the fish can adjust to their new environment, hone their burial skills, begin pigment change, and recover from the stress of transport to the release site, all in the absence of predation. However, there have been indications that the cages attract the predatory green crab, Carcinus maenas. Studies conducted at the release site in the Hampton-Seabrook Estuary, New Hampshire, USA, determined that green crab abundance was significantly higher (ANOVA, p < 0.01) on cages containing fish than on empty cages, proving that acclimation cages containing flounder do attract green crabs. In addition, when empty acclimation cages were deployed, crab densities significantly increased (ANOVA, p < 0.01) in the vicinity of the acclimation cages and continued to increase each day, indicating that green crabs are also attracted to empty cages. Thus, although acclimation cages are a necessary tool that allows the stocked fish to adjust to their new environment, they also may be a detriment if they attract predators to the site. Using active adaptive management, release strategies must be reevaluated to reduce such predator-prey encounters, thereby improving fish survival.

Responsible Approach to Marine Stock Enhancement: An Update

“…Key release parameters that affect survival of hatchery fish released into the wild include fish size-at-release (e.g., Hager and Noble, 1976;Tsukamoto et al, 1989;Svåsand and Kristiansen, 1990b;Leber et al, , 1997Leber et al, , 1998Leber et al, , 2005Lorenzen, 2000), release season (e.g., Bilton et al, 1982;Leber et al, 1997Leber et al, , 1998Hines et al, 2008), release habitat and microhabitat (e.g., Leber et al, 1998;Gardner and Van Putten, 2008;Hines et al, 2008), release magnitude (Brennan et al, 2008;Hines et al, 2008), transport and release associated stress (Sulikowski et al, 2005;Fairchild et al, 2009), and acclimatization at release sites (Brennan et al, 2006;Fairchild et al, 2008;Hervas et al, in press). These and similar experimental studies have shown that choices made about release parameters can result in significantly different mortality rates of released hatchery fish.…”

Successful stocking of a depleted species, spotted halibut Verasper variegatus, in Miyako Bay, Japan: evaluation from post-release surveys and landings

“…Recent studies have demonstrated that conditioning or learning of reared fish before release could improve behavioral skills such as feeding, burying and predator avoidance (Hossain et al 2002, Fairchild & Howell 2004, Arai et al 2007, resulting in higher survival of juveniles just after release . However, some negative impacts of conditioning processes have been observed, such as attraction of predators during on-site acclimation using cages (Fairchild et al 2008). Although conditioning prior to release may improve the slightly delayed feeding of spotted halibut juveniles just after release, the fact that juveniles could adapt to the release habitat within 1 wk after release, and subsequently undergo a high incidence of recruitment to the fishery, might indicate that conditioning before release may not be necessary under near-optimal biotic/abiotic release conditions.…”

Effects of moving acclimation cages before release of cultured fish: alternate release strategies for a juvenile winter flounderPseudopleuronectes americanusstock enhancement effort

“…However, the cages also can be detrimental by attracting predators to the release site. In a recent study, Fairchild, Rennels and Howell (2008) compared green crab density when cages were absent, present but empty and present and containing flounder. They found that crab density increased dramatically from 1.8 to 4.1 crabs 50 m −2 when empty cages were deployed.…”

“…Site A is a well‐studied location in the Hampton River that has been used as a release site for previous winter flounder stock enhancement studies. Site B is approximately 250 m downriver from Site A and has been examined as an alternate release site (Fairchild, Rennels et al 2008). Both sites are sandy areas devoid of any macroalgae or structure, about 4 m deep at mean high water, have high salinity and dissolved oxygen, and naturally occurring wild juvenile winter flounder populations (Fairchild, Sulikowski, Rennels, Howell & Gurshin 2008).…”

“…Despite many of the positive attributes benthic acclimation cages afford cultured winter flounder, they are not effective since predatory green crabs are attracted to them (Fairchild, Rennels et al 2008), and moving the cages adversely affects the fish. Because there are significant benefits to acclimating winter flounder in situ before release, it is worthwhile to try other cage designs.…”