Natural Mortality and Size Structure of Introduced Blue Catfish in Virginia Tidal Rivers

In the 1970s and 1980s, blue catfish (Ictalurus furcatus) were introduced to the tidal rivers of Virginia. Current abundances and uncertainty about population characteristics of blue catfish generated concern for other economically important and imperiled species. We estimated natural mortality and size structure of blue catfish for four tidal river systems (i.e., James, Mattaponi, Pamunkey and Rappahannock). Using common empirical estimators with pooled data from the period 2002-2016, we calculated five estimates of natural mortality. Proportional size distributions were used to examine changes in size structure over time. Maximum observed age of 25 years indicated mature populations. Estimated mean instantaneous natural mortality (M) from five empirical estimators ranged from 0.13-0.19 in the four rivers. Temporal trends in size structure differed among rivers, likely due to differences in stocking timing and riverine productivity. Proportions of memorable and trophy size blue catfish appear to have recently declined in three of four rivers, but size-structure indices demonstrated continued viability of the James River trophy fishery. This study provides blue catfish mortality estimates to support development of future stock assessment models and management strategy evaluations. Understanding the size structure of these populations will help resource managers gauge the prevalence of large blue catfish relative to historical conditions and provide information on angling opportunities and size-based trophic interactions.

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