Scholten, G. D., and P. W. Bettoli. 2005. Population characteristics and assessment of overfishing for an exploited paddlefish population in the lower Tennessee River. Transactions of the American Fisheries Society 134:1285-1298.
Abstract
Paddlefish Polyodon spathula (n = 576) were collected from Kentucky Lake, KY-TN, with experimental gillnets in 2003-2004 to assess population characteristics and the likelihood of commercial overfishing. Additional data were collected from 1,039 paddlefish caught by commercial gillnetters in this impoundment. Size and age structure have been reduced and annual mortality has tripled since the most recent study in 1991. Thirty-seven percent of the fish collected in 1991 were older than the maximum age we observed (age 11) and annual mortality for age 7 and older paddlefish in 2003 was high (A = 68%). Natural mortality is presumably low (<10%) for paddlefish; therefore, exploitation in recent years was high. Estimates of total annual mortality were negatively related to river discharge in the years preceding each estimate. The number of paddlefish harvested since 1999 was also negatively related to river discharge because gill nets cannot be easily deployed when discharge exceeds ~ 850 m3/sec. Large females spawn annually because all females longer than 1,034 mm eye-fork length (EFL) were gravid. No mature females were protected by the current 864 mm minimum EFL limit. At a low natural mortality rate, simulated flesh yields increased 10-20% with higher size limits when exploitation was high (40-70%). Even at low levels of exploitation (u ≥ 21%), Spawning Potential Ratios (SPR) under the current minimum size limit fell below 20%. If the size limit was raised to 1,016 mm EFL, the population could withstand up to 62% exploitation before the SPR fell below 20%. An analysis of annual mortality caps indicated the best way to increase average size of harvested fish is to increase the minimum size limit. Recruitment overfishing probably occurs during drought years; however, variation in river discharges has prevented the population from being exploited at unsustainable rates in the past.