MEPS 479:163-175 (2013)  -  DOI: https://doi.org/10.3354/meps10191

ABSTRACT: Juvenile Japanese flounder Paralichthys olivaceus inhabit shallow sandy areas and consume chiefly mysids. Hatchery-reared P. olivaceus (ca. 100 mm in total length) released in stock enhancement programs also consume mysids. To examine whether stocking is implemented within the available carrying capacity, we assessed the feeding relationships among fishes, based on the stomach contents of fishes collected off the coast of Fukushima, Japan. Similarity in diet suggested that 10 species, particularly the poacher Occella iburia and nibe croaker Nibea mitsukurii were potential competitors of P. olivaceus juveniles for food. Large inter-annual variability in the abundance of these competitors was observed, suggesting variability in their consumption of mysids. The predominant mysid Orientomysis mitsukurii was abundant every year, and growth rates of wild P. olivaceus, estimated from otolith microstructure, were mostly high (>1 mm d^-1), even in the year when wild P. olivaceus were highly abundant. In our statistical model, abundance of mysids and consumption of mysids by fishes significantly affected the growth rates of wild P.olivaceus but only accounted for a small proportion (i.e. explained 2.2 and 2.4% of variance, respectively) of the total compared to the body size of juveniles (30.0%) and bottom water temperature (4.5%). These results suggest that the productivity of mysids is usually high enough to support the production of mysid consumers, but exceptionally high abundances of wild P. olivaceus or other competitors can reduce the available carrying capacity. In such a situation, stocking should be restricted so as not to reduce productivity of wild fishes.

KEY WORDS: Flatfish · Mysids · Growth rates · Linear models · Stock enhancement · Carrying capacity