Size- and Sex-Selective Fishing Leads to Maladaptive Sex Ratio Adjustment by a Sex-Change Shrimp

Although selective fishing often causes plastic life history changes of target species, there is little evolutionary perspective on those changes. We examined a facultative sex-change of the protandrous shrimp (Pandalus latirostris) to the skew of population structure by unintentional female-selective fishing due to a favoring for large size. Our 12-year survey showed that annual fluctuation of the sex ratio was diminished in the maturing season by occurrences of females that changed sex early or males that postponed sex-change. Both field observation and laboratory experiment confirmed that those sex-changes were induced by the skew of age and sex structures, suggesting sex ratio adjustment. Sex ratio adjustment is not sufficient, however, because large individuals that can change sex have been selectively removed by fishing in the previous year. Moreover, such adjusted sex ratio is skewed again by subsequent fishing acting as female-selective, suggesting that the sex ratio adjustment often becomes rather maladaptive. A consideration of facultative changes in the life history of harvested animals would be more important than our recognition to conserve wild animal resources.