Thermal Limits On The Ocean Distribution Of Steel head Trout (Oncorhynchus mykiss)

We describe sharp thermal limits to the ocean distribution of steelhead trout (Oncorhynchus mykiss) which restrict the distribution of steelhead to only a small fraction of the area of the North Pacific otherwise available for grazing. Maximum likelihood estimates indicate that (1) steelhead exhibit a strong response to temperature in all regions of the North Pacific ocean and in all seasons ofthe year; (2) both upper and lower thermal thresholds exist; (3) although the thermal limits have been largely stable over the last 40 years, the temperature defining the southern edge of the steelhead distribution exhibits statistically significant fine-scale differences between regions «2°C) and between decades «0.5°C). As a result of this strong thermal control, steelhead are distributed in a narrow north-south band stretching across much of the width of the Pacific. Projections of the degree of ocean warming resulting from greenhouse gas induced climate change suggest that the ocean distribution of steelhead may shift far enough north under a 2xC02 climate that large areas of currently occupied ocean habitat could lie outside the range of these thermal limits by the middle of the next century.