Bomb Radiocarbon Validates Age and Long-Term Growth Declines in American Plaice in the Southern Gulf of St. Lawrence

Abstract

The growth rate and size composition of American Plaice Hippoglossoides platessoides in the southern Gulf of St. Lawrence has changed markedly over the past 40 years. The size at age recorded in commercial fisheries and research survey data has declined, particularly through the mid-1980s. Females of the 1969 cohort grew to an estimated maximum length of 59 cm (males, 42 cm); the 1994 cohort grew to a much smaller maximum of 37 cm (29 cm for males). There is no indication of any reverse trend in growth or size at age in the 2000s, despite reduced harvests and warmer water temperatures. Incorrect aging methods are not the reason for the observed growth trends. Ages were determined by counting the growth increments visible on the surface of whole otoliths, and there is no bias in ages determined by this method (as opposed to those from thin-sectioned otoliths). The accuracy of ages determined by reading whole otoliths was validated to at least 27 years (±2 years) by bomb radiocarbon assays of 39 otoliths collected between 1971 and 1986. While commercial fishing may have contributed to past declines in plaice growth, ongoing natural mortality may be keeping size at age and the growth rate low despite the recent increases expected from low fishing mortality and higher water temperatures.

Received August 14, 2012; accepted November 14, 2012

ACKNOWLEDGMENTS

Financial support for radiocarbon isotope assays was provided by the Species at Risk Program (DFO Gulf Region). Diana González-Troncoso provided unpublished growth parameters for American Plaice populations in the North Atlantic. We thank Anna Dorey and Warren Joyce for technical assistance in preparing, micromilling, and imaging the otoliths. We also thank the staff who contributed age determinations, particularly Sharon Ryall and Isabelle Forest who served the program for over 30 years. Two anonymous reviewers provided helpful comments on an earlier draft of this work.