Environmental Effects on Recruitment, Growth, and Vulnerability of Atlantic Herring (Clupea harengus harengus) in the Gulf of Maine Region

Atlantic herring (Clupea harengus harengus) recruitment in the Gulf of Maine since 1947 has varied by a ratio of 20 to 1. Since heavy fishing began in the mid-1960's, recruitment has fluctuated by only a factor of 9 to 1. The greatest fluctuations in recruitment, therefore, historically occurred in the absence of high fishing mortality. Recruitment predictions and understanding of the causes of fluctuations are extremely important, since strong year classes traditionally have sustained the herring fisheries in the Gulf of Maine. The effect of environmental variables (particularly temperature) on herring recruitment and growth were examined in detail. Vulnerability of Gulf of Maine herring in response to moon phase is also considered. Indices of abundance of Atlantic herring in the Gulf of Maine were calculated for three time periods using three different procedures. Indices of abundance for the periods of 1915–67 and 1951–81 indicated that productivity, or amount of recruitment per amount of spawning stock, was positively related to temperature or other factors (e.g. food availability) related to temperature at intermediate to high levels of spawning stock biomass. For the shorter and most recent time period (1965–81), abundance was calculated by virtual population analysis and an attempt was made to relate temperature effects during several periods in the first year of life to recruitment levels at age 2. The mean, maximum, and minimum water temperatures during (1) September–December (spawning – early larval development), (2) January–April (overwintering and late larval development), and (3) May–August (postlarval) periods were correlated with abundance. Significant effects of mean and minimum temperature during period 2 and minimum temperature during period 3 were observed, suggesting that environmental influences on determination of year class strength occur during late larval – early juvenile phases. Possible reasons for the discrepancy between the long-term analyses bases on abundance indices and the more detailed analyses using population size estimates based on virtual population analysis are discussed. An indication of environmental limitation is also shown by density-dependent growth. Growth appears to be related to both age 2 abundance and summer water temperature. When abundance is great, its effect overcomes the positive effect of temperature (or other factors indicated by temperature). The environment also alters the availability and vulnerability of herring to the inshore fisheries. Young herring are more available and vulnerable to fixed gear during the dark phase of the moon. This effect is pronounced when abundance is low.