Rebuilt tag records suggest Atlantic menhaden natural mortality rate is half prior estimates

Menhaden tags were metal, recovered from harvested fish by magnets at the processing plants. A tag that never comes back is ambiguous: the fish may have died of something other than fishing, or it may have been landed with the magnet missing the tag ^[8]. Detection varied widely among plants and over time ^[8]. Models that let tag-detection efficiency vary by location and by month matched the recovery records better ^[10].

Ault and Luo worked on the records for roughly three years, checking release and recovery counts, rebuilding historical fishing effort and examining plant detection to produce two separate reconstructions of the experiment ^[7]. Both reconstructions gave natural mortality near 0.50 per year, and simulations that varied the assumptions kept returning similar best-fitting values ^[11]. "Our investigation identified discrepancies involving the numbers of released and recovered tagged fish, reconstruction of historical fishing effort, treatment of tag-detection efficiency at processing plants, and aspects of the statistical model," said Ault ^[17].

The three competing numbers describe the same fish. 0.50 is about 43 percent of 1.17 and about 54 percent of 0.92 ^[21]. Read as instantaneous annual rates, they imply very different survival: exp(-M) leaves about 31 percent of a year class alive after a year at 1.17, 40 percent at 0.92 and 61 percent at 0.50, before any fishing ^[22]. Run that out a decade and 1.17 leaves roughly eight fish per million at ten years, against about one in 150 at 0.50 ^[23]. Ault and Luo report that their estimate lines up with earlier research and biological evidence showing Atlantic menhaden historically lived at least 10 years ^[12].

The study does not measure how menhaden die now. The tags went into the water between 1966 and 1971, about 55 years before publication ^[24], and the authors present the result as a correction to a historical estimate ^[13]. Their argument for why the correction matters is procedural: set the rate too high and fishing's impact is underestimated, the breeding population is overestimated, and the harvest advice comes out optimistic ^[20].

"If natural mortality is high, fishing appears to account for a smaller share of population change. If it is closer to 0.5, as this study and earlier historical estimates suggest, fishing may account for a larger share of the losses," Ault said ^[18]. "For a species that is both heavily harvested and ecologically central, that distinction matters not only to the menhaden fishery but to the many managed predators that rely on menhaden," he said ^[19].