Clues for Recruitment Variability from Historical Data: Invariant Patterns of Larval Growth for Subtropical Cutlassfish Trichiurus Japonicus

The early life growth rates are an essential determinant for recruitment success, providing important implications for fisheries management. However, such critical information is insufficient for many exploited subtropical species, which often display prolonged spawning seasons and are harvested all-year round. The cutlassfish Trichiurus japonicus provide valuable fisheries in Taiwan in all seasons, thus their recruitment variability is of great concern. Using historical long-term monthly samples collected from two Taiwan coasts (about 25°N from NW and NE Taiwan, 1996-present), we investigated temporal variation in growth for larval cutlassfish (age =10-40 days) by month, years, and decades. Larval growth was estimated by measuring individual total lengths and otolith daily ages. Interestingly, we found that after excluding some outliers growth rates were virtually invariant among months, years, and decades. Also, although there were not enough data points, growth rates appeared to be negatively correlated with relative abundance, implying potential density-dependent effects. Subsequently, we estimated egg size and recruitment strengths by months and related these to larval growth patterns. Results from this study not only enhance the understanding of early life history ecology but promote an in-depth understanding of long-term climate effects on subtropical fish populations.