Harvest of Mississippi River sturgeon drives abundance and reproductive success: a harbinger of collapse?

Abstract

Within harvested populations, relationships between harvest intensity and reproductive responses are typically unclear, rendering regulatory decisions difficult. Harvest of the commercially important shovelnose sturgeon (Scaphirhynchus platorynchus) is increasing in the upper Mississippi River; standardized seasonal sampling revealed that adult abundance is declining. Relative density of annual cohorts varied negatively with historical harvest intensity (r 2 ¼ 0.84), suggesting that removal of mature adults is reducing the contribution of cohorts to population density. The results of simulation modeling suggest that this currently unregulated fishery is experiencing both growth and recruitment overfishing. Further, the current proposed multi-state minimum length regulation was insufficient to maintain a sustainable stock. Only a more conservative minimum length limit (685 mm) produced yields that were sustainable at the current level of mortality and provided room for the fishery to grow. The annual mortality rate of the sympatric, federally endangered pallid sturgeon (S. albus) was similar to that of the shovelnose sturgeon population, raising concerns that harvest-induced mortality is affecting this congener's vital rates.

Maturation and Reproduction of Shovelnose Sturgeon in the Middle Mississippi River

“…However, our results (males ¼ 8 years; females ¼ 9 years) show evidence that these populations are now reaching maturity at later ages. Harvest occurs on shovelnose sturgeon in this reach (Colombo et al 2007a). Loss of rapidly growing, earlymaturing fish to selective harvest (especially ripe females) may cause late-maturing, slow-growing individuals to be more prevalent (Ernande et al 2003).…”

“…Of the 27 extant species, all are characterized by limited adult abundance and most are threatened (Billard and Lecointre 2001;Pikitch et al 2005). The Caspian Sea sturgeon fishery has long produced the majority of the caviar traded internationally, but with the collapse of this and other European and Asian sturgeon fisheries, increased pressure has shifted towards the smaller North American species, such as the shovelnose sturgeon Scaphirhynchus platorynchus (Quist et al 2002;Pikitch et al 2005;Colombo et al 2007a). With much commercial effort being diverted towards the shovelnose sturgeon population in some reaches of the Mississippi River and continued habitat alterations occurring throughout the entire river system, a thorough understanding of its life history is needed (Keenlyne 1997;Colombo et al 2007a).…”

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

“…Harvest of shovelnose sturgeon for roe (i.e., caviar) was allowed in eight states in the central US (Tennessee, Missouri, Kentucky, Arkansas, Indiana, Iowa, Illinois, and Wisconsin; Koch and Quist 2010). It has been estimated that pallid sturgeon mortality rates were about three times greater where shovelnose sturgeon were harvested (Colombo et al 2007;Killgore et al 2007b). Thus, the SOA provision was implemented to prevent suspected bycatch of the pallid sturgeon (Bettoli et al 2009).…”

Reproductive biology of middle Mississippi River shovelnose sturgeon: insights from seasonal and age variation in plasma sex steroid and calcium concentrations

“…The contradiction in estimates of age at maturity between earlier and more recent studies may result from a reduction in growth rate across all age classes and a shift in the age-frequency distribution toward older fish in the MMR shovelnose sturgeon population between (Tripp, 2007. A shift toward older age at maturity in MMR female shovelnose sturgeon may also be a consequence of increased harvest during recent years (Colombo et al, 2007a). (Tripp, 2007), which would be expected to favor maturation at a later age and larger size (Law, 2000;Conover and Munch, 2002;Ernande et al, 2004).…”

“…Shovelnose sturgeon are heavily harvested from the middle Mississippi River (MMR; the portion of the river between the mouth of the Missouri River and the Ohio River confluence); strong associations between harvest and annual cohort strength suggest that Mississippi River shovelnose sturgeon are experiencing recruitment overfishing and may be at risk of collapse from overexploitation (Colombo et al, 2007a).…”