Effects of harvest and length limits on shovelnose sturgeon in the upper Wabash River, Indiana

Abstract

Shovelnose sturgeon Scaphirhynchus platorynchus are one of the few sturgeon species that currently support sustainable commercial harvest. However, harvest closures for many Eurasian sturgeons have resulted in increased exploitation of this fishery, thereby raising concerns about the sustainability of shovelnose sturgeon resources. As a result, the maintenance of self-sustaining shovelnose sturgeon populations will require the estimation of appropriate harvest levels. This study used an age-structured population model to examine the effects of harvest (u ¼ 0.15-0.75) and length restrictions on population abundance, mean length-at-harvest, biomass, yield, and reproductive potential of female shovelnose sturgeon in the upper Wabash River, Indiana. Model simulations for four hypothetical length-restriction scenarios (610-to 813-mm reverse slot limit, and a 610-, 635-, and 660-mm minimum length limit) were compared to outputs with no restriction. All population parameters within each length-restriction scenario declined with increases in harvest level. For each harvest level, all population parameters increased as length limits became more restrictive. The reverse slot limit and 610-mm minimum length limit provided adequate protection to allow population parameters to increase through an annual harvest level of 0.55. However, these length restrictions were not sufficiently conservative to warrant implementation due to their similarity to length-at-maturity of female shovelnose sturgeon. The implementation of a 635-mm minimum length limit would protect female shovelnose sturgeon from harvest rates >0.75, allow 92% of the females to remain available for harvest, and minimize short-term (<30 years) declines in yield. Further, sensitivity and robustness analyses suggested that the 635-mm minimum length limit would allow population parameters to increase even at the worst-case scenario. As a result, the 635-mm minimum length limit was recommended as the most appropriate regulation to promote conservation and sustainable harvest of shovelnose sturgeon in the upper Wabash River.

Population viability analysis of the Danube sturgeon populations in a Vortex simulation model

“…Population viability was most strongly influenced by changes in the age at which females reach maturity, in the mean fecundity, egg to age 1 natural mortality and juvenile mortality (age groups 1-4), as well as by sex ratio, spawning frequency and adult mortality. These results are in accordance with PVAs performed on North American sturgeon species (Pine et al 2001;Beamesderfer et al 2007;Kennedy and Sutton 2007), that have found that the most influential parameters were egg mortality, age at maturity of females, spawning frequency and the overall natural mortality. Kennedy and Sutton (2007) estimated that the egg mortality was the single most influential population parameter, and an increase of 0.0001% units in mortality rate (from 0.9998 as the average value) caused as much as a 95% decline in population abundance and biomass.…”

“…These results are in accordance with PVAs performed on North American sturgeon species (Pine et al 2001;Beamesderfer et al 2007;Kennedy and Sutton 2007), that have found that the most influential parameters were egg mortality, age at maturity of females, spawning frequency and the overall natural mortality. Kennedy and Sutton (2007) estimated that the egg mortality was the single most influential population parameter, and an increase of 0.0001% units in mortality rate (from 0.9998 as the average value) caused as much as a 95% decline in population abundance and biomass. Bajer and Wildhaber (2007) have found that the egg, juvenile and young adult mortality were the most influential parameters on the population growth rate, while the fecundity was a less sensitive parameter.…”

“…It was determined in both the present study and by Kennedy and Sutton (2007) that the mortality of eggs and the youngest age classes were much more influential than the mortality of adult individuals. On the other hand, Pine et al (2001) have established that the population of A. oxyrinchus desotoi was sensitive to even small changes in the adult mortality, and Boreman (1997) and Heppell (2007) have claimed that a small reduction in mortality of the adults could balance a relatively large increase in egg to age 1 mortality.…”

Considering a species-loss domino-effect before endangered species legislation and protected area implementation

“…One study modeled the effects of harvest on shovelnose sturgeon in the upper Wabash River (US) where shovelnose sturgeon are not protected by the SOA provision. They found harvest rates of female shovelnose as low as 15 % negatively affected abundance, yield, and reproductive potential (Kennedy and Sutton 2007). It is unlikely that fisheries for shovelnose sturgeon in the unprotected areas will meet demand in a sustainable manner without very strict regulations by state agencies bordering the rivers, which do not yet exist.…”

“…For example, there is no commercial harvest in the upper Wabash River (Fig. 3) and this region is known to provide important spawning grounds for shovelnose sturgeon (Kennedy and Sutton 2007). States could consider river reaches of similar importance for reserve establishment.…”