Quantitative Evaluation of Post-Release Mortality Using Salt Pond Mesocosms: Case Studies of Hatchery and Wild Juvenile Tiger Puffer

Abstract

Quantitative approaches in release trials are typically used to evaluate fish quality for release. However, release trials under natural conditions include masking factors in their experimental designs. To streamline the experimental design of release, we used a semi-natural environment: a salt pond mesocosm (5,300 m²). This article reviews recent findings related to comparison of post-release mortality, behavior, and physiological conditions between wild and hatchery-reared juveniles of the tiger puffer (Takifugu rubripes), which were released into the mesocosm with or without predators (sea bass, Lateolabrax sp.). Results of release trials in the mesocosm suggested that hatchery-reared juveniles were more easily preyed upon than wild juveniles, and that predation occurred shortly after release, decreasing thereafter. Results revealed differences in swimming depth after release and different accumulations of tetrodotoxin (TTX) in reared and wild fish. Predation shortly after release and behavioral defects in hatchery-reared juveniles, such as changes in swimming behavior and feeding behavior, might be the main causes of mortality in this species' stock enhancement programs.

Keywords:

• enclosure
• predation
• fish quality
• behavior
• tetrodotoxin

ACKNOWLEDGEMENTS

We would like to express our gratitude to Mr. Tetsuo Yamazaki, chief of Stock Enhancement Technology Section of National Research Institute of Fisheries and Environment of Seto Inland Sea, FRA, for providing the opportunity to conduct this study. We also wish to express our sincere thanks to the entire staff of the National Center for Stock Enhancement Yashima Station, including Mr. Akio Iwamoto, the station's Director, for their cooperation. Financial support was received from the MEXT Special Education Research Collaborative Project Japan and Japanese Grants-in-Aid for Promotion of Scientific Research to YS and TT. We are grateful to two anonymous referees for their critical review of an earlier version of this manuscript.

Notes

^a p < 0.001,

^b p < 0.01).

*¹Recaptured individuals from the feeding habit investigation are included in the survival rate of release trial II.

^a Significant difference between hatchery-reared and wild juveniles by χ ²-test ( ^a p < 0.001).

*²Growth rate = (final standard length – initial standard length)/(initial standard length × experimental term(day)) × 100. Significant difference between hatchery-reared and wild juveniles by Mann Whitney U-test

^b p < 0.05).