The Delaware River American Shad Stock: And the Hypothesis That Striped Bass Predation Controls Shad Abundance

Our assessment of the Delaware River American shad stock and its fisheries has recently been concluded, using an index of relative abundance from 1925 through the present with other data. The assessment was conducted by a multi-state fishery cooperative focused on the Delaware River, the only undammed major river on the East coast of the United States. Our conclusion is that the stock’s reproduction is healthy and its fishery is sustainable. In the late nineteenth and early twentieth century, the Delaware River and Bay produced the largest shad landings in the country.  American shad migrate up to 300 miles upriver from the mouth of Delaware Bay into New York state waters. Young-of-year production has been monitored annually since 1980, using two beach seine surveys. There is a statistically significant positive trend in YOY catch-per-haul, stemming largely from low indices in the earliest years. Plots of the young-of-year index as a function of the adult index show that reproductive output of this stock has been stable, even when adult run size declined to the lowest levels observed since 1980. Adult abundance was low in mid-century due to annual anoxia in 30 miles of River from Philadelphia to Wilmington.  Abundance increased in the 1980s and early 1990s, then declined steeply. Managers shut down the coastal gill net fishery by 2005, assuming that the fishery was the cause of the coastwide decline, but the stock did not respond. We tested the hypothesis that striped bass predation could affect shad abundance, and found a highly significant negative correlation between relative abundances of striped bass and shad. In the last few years, bass abundance has declined coastwide and shad abundance in the Delaware has risen noticeably. The two extant hypotheses are that water quality and striped bass predation have exerted major influences on the population dynamics of Delaware River shad.