Apparent predation of juvenile coho salmon (Oncorhynchus kisutch) by prickly sculpins (Cottus asper) is an artefact of trapping methodology

Abstract

Freshwater sculpins often inhabit the same waterways as juvenile salmonids and may impact the survival of juvenile salmonids through predation on early life-history stages. In the present study, the stomach contents of 2302 individual Cottus asper, a freshwater sculpin, collected from Auke Lake, Alaska, were examined during the boreal summer of 2000 to determine if C. asper are important natural predators of juvenile coho salmon (Oncorhynchus kisutch), and to explore possible trap bias of gear used in preliminary diet studies. The diet of sculpins collected in confining traps was compared with the diet of sculpins collected in nets. Significant predation on pre-smolt coho salmon by trapped sculpins, but none by netted sculpins, was observed. This result provides strong evidence of trap bias in the observed diet of C. asper. The remainder of the diet of trapped sculpins also differed significantly from that of netted sculpins. Significantly more trapped sculpins had eaten plant material and fish, whereas significantly more netted sculpins had consumed molluscs. Finally, sculpin diet was correlated with sculpin size, which may influence predation on other salmonid life-stages. These results expand our understanding of prickly sculpin diet and show that they are not important predators of juvenile coho salmon. These findings also demonstrate the importance of assessing the potential bias of collection gear and sampling techniques.

Spatial and Temporal Patterns in the Diet of Striped Bass in Chesapeake Bay

“…We hoped that our multigear sampling approach may have 924 masked any gear effect, thus, averaging out any gearbias effect of gear type on diet indices. Gear type or capture methods can influence diet indices of fish (Merriner 1975;Tomaro 2006). A comparison of the diet by gear type may be warranted to provide a basis for assessing the potential bias of collection gear in our study and the evaluation of other published food habits studies of striped bass in Chesapeake Bay.…”

Natural Reproductive Success and Demographic Effects of Hatchery-Origin Steelhead in Abernathy Creek, Washington : Annual Report 2008.

“…Figure 3.3-Box plots of plasma osmolality levels for NOR and HOR steelhead migrants captured at the Abernathy screwtrap and subjected to a 24 hour seawater challenge. A seawater challenge was conducted for each of the three hatchery release blocks in , 2005, 2006. Trials 2 & 3 in 2007 were from the second release block.…”

“…In November 2004, the HOR juvenile steelhead (N ~ 20,000) produced from the 2001 collection-year NOR adults were given a CWT and an adipose clip, with 1000 also receiving a PIT tag in January 2005. In November 2005, 2006, 2007, the HOR juvenile steelhead (N ~ 20,000) produced from the NOR, F 1 , and F 2 HOR returning adults were given a CWT and a adipose clip, with 1000 also receiving a PIT tag in January 2006, 2007, 2008and 2009. From 2003through 2008 tagged HOR steelhead smolts were released into Abernathy Creek from AFTC in three forced releases.…”

“…We characterized dietary overlap between hatchery and wild fish as both smolts and non-migratory summer juveniles by using the percent dietary overlap index (% overlap = Σ[minimum(P ij ,P ik )], where P ij and P ik are the mean percent abundance by number of prey item i in the stomach of group j and k; Schoener 1970). During the second and third HOR steelhead release blocks of 2007, we examined a subset of steelhead stomachs immediately after capture to account for any trapping bias that may overestimate fry predation (Tomaro 2006).…”

Seasonal foraging and piscivory by sympatric wild and hatchery-reared steelhead from an integrated hatchery program

“…Because salmonids confined inside the screw trap for up to 24 h can experience an unsheltered environment with unnatural predator and prey densities (Tomaro 2006), we also lavaged a subsample of steelhead (N=92) immediately upon entering the trap after the second and third hatchery releases of 2007. The stomach contents of all steelhead were immediately preserved in a 75% ethanol and rose bengal solution.…”