Phenological shifts in hatch timing of northern shrimp Pandalus borealis

The northern shrimp Pandalus borealis reaches the southern limit of its distribution in the Gulf of Maine, USA. Previous studies have revealed geographic clines in northern shrimp population parameters (growth, maturity, longevity) which co-vary with environmental temperature, and laboratory studies have confirmed temperature effects. In this study, field data were used to investigate reproductive phenology of northern shrimp in the Gulf of Maine during 1980 to 2011 in relation to ocean temperatures. Timing of the annual shrimp hatch was estimated by sampling commercial catches during the brooding and hatching period. Hatch timing metrics were regressed against composite environmental variables derived from principal component analysis of sea surface temperature and bottom temperature anomalies. Shrimp population indices (spawner abundance and mean size) were included as covariates. Shifts have occurred in timing of hatch initiation (earlier) and completion (later), with the overall result of a longer hatch period in recent years. The midpoint of the hatch period has varied without trend. All hatch timing metrics were significantly related to temperature variables, several of which have warmed significantly during the study period. In the short term, phenological shifts in hatch timing of northern shrimp may be beneficial because they effectively increase the window of opportunity for larvae to encounter good survival conditions. The continued warming predicted for the Gulf of Maine will likely produce further changes, some of which may be in unexpected directions.