Effects of density reduction on age-specific growth of stream-dwelling Brown Trout

Density-dependent growth has been well documented among stream-dwelling Brown Trout Salmo trutta populations. In Spearfish Creek, South Dakota, biomass of adult Brown Trout (>200 mm) is about three times greater than that reported for similar Black Hills streams, whereas the mean length of adult fish is about 30% less. Here, we evaluate density reduction as a management tool for improving the growth rate of stream-dwelling Brown Trout. We compared age-specific growth of wild Brown Trout in stream sections receiving 50% reductions in fish abundance (removal sections) to that of fish in sections containing natural densities (control sections). Annual growth in length and weight of older Brown Trout (age > 2) was greater in removal sections compared to control sections, particularly among fish between 230 and 280 mm TL (ages 3–4). We attribute the growth response to constraints imposed by food availability and the strong feeding hierarchies, characteristic of larger (older) Brown Trout. Data collected from PIT-tagged fish that were recaptured 2 years after fish removal efforts revealed that growth responses owing to density reduction likely did not extend beyond 1 year. Nonetheless, among cohorts that showed improved growth after the first year, we found that their growth advantage was sustained into the second year. At removal sections, age-3 and older Brown Trout that experienced greater growth after 1 year were larger than fish from control sections 2 years later at age 5 and older. Identification of cohort(s) for which density-dependent growth is most pronounced could help to focus efforts on targeted reduction of specific fish sizes/ages that minimize the cost–benefit ratio of fish removal efforts.