Population dynamics of the shortfin mako, Isurus oxyrinchus, in the northwest Atlantic: An examination of food habits, movement and habitat, survival, and population size

Until recently, fish and invertebrate species with high economic value were the main focus for stock assessment and fishery management plans. Though less economically important, elasmobranchs are being increasingly targeted by both commercial and recreational fisheries. In order to better understand the current status of the shortfin mako population in the northwest Atlantic, this thesis examined four aspects of shortfin mako population dynamics resulting in five manuscripts (chapters). The first chapter generated regression equations that allowed for back-calculation of bluefish (an important prey species) sizes from digested remains. The second chapter revisited the food habits and daily ration of shortfin mako and offered a historical perspective as comparison. Regression equations from chapter one were used to back-calculate prey bluefish sizes. It was found that makos still feed mainly on bluefish, which made up 92.6% of the diet by weight, and may exert a significant top-down pressure on the bluefish population. The third chapter deployed 4 pop-up archival transmitting (PAT) satellite tags to track mako movement and environmental preferences. Four juveniles, tagged off the northeast U.S coast moved on average 67 km/day in random excursions, preferring water temperatures ranging from 20 to 23°C, and depths from 0 to 10 m. The fourth chapter used tag-recapture data to estimate survival of the mako population. The model with survival as a constant rate parameter had the best overall fit and returned an annual survival rate of 0.789 y-1. The final chapter of this work analyzed U.S. and Canadian observer and longline data to estimate relative abundance and population size in the North Atlantic. Shortfin mako population biomass was estimated to be right around its carrying capacity of 200,000 metric tons, or approximately 3,000,000 individuals in the entire North Atlantic. The results from this project offer new biological data and population parameter estimates that can be used as inputs to stock assessments, and as an aid in setting current management guidelines for the shortfin mako in the northwest Atlantic