American Shad Migration in the Delaware River 2010

Pennsylvania Fish and Boat Commission (PFBC) biologists from Area 5 have completed two surveys at Raubsville (RM 178.9) and Smithfield Beach (RM 218.0) for adult American shad in the Delaware River. Both surveys generally collected similar information on the adult shad migration in the Delaware River; however, their primary purposes were different. The Raubsville survey is designed to provide a relative index of abundance of adult shad for judging the strength of the returning run, while the Smithfield Beach survey focuses on obtaining artificially fertilized brood stock eggs for supporting PFBC’s restoration plan of returning a self-sustaining wild shad run into the Lehigh and Schuylkill Rivers.

The Raubsville index was generated from electrofishing once a week for adult American shad beginning the first week in April (04/08/2010) and terminating the first week of May (05/06/2010). A total of 110 shad, 27 females and 83 males, were collected, for an overall average catch rate of 22.0 shad/hour. Other than the second week of the survey, weekly catch rates were excellent ranging from 17.1 – 39.0 shad per hour. The poor catch rate (1.9 shad/hour) observed during the second week of the survey was most likely related to a temporary lull in passage of shad early in the season in that particular reach of river.

Biological data from shad captured at Raubsville showed that sizes and weights ranged from 415 - 563 mm total length or 16.3 - 22.2 inches (females: 17.1 – 22.2 inches; males: 16.3 – 21.6 inches) and weight ranged from 654 – 2,253 grams or 1.4 – 4.9 pounds (females: 2.4 – 4.9 pounds; males: 1.4 – 4.3 pounds). Scales and otoliths were also collected for estimation of age, but they have not yet been processed. Interestingly, the sex ratio was highly skewed towards males (3 males for every one female). The early termination of the Raubsville index may have caused the index to underestimate returning females, which tend to immigrate later than males.

Figure 1. Catch-per-unit-of-effort for American shad in the Raubsville electrofishing survey during the 2010 migration.

The Raubsville index represents the re-initiation of an historical survey conducted from 1997 through 2001 in the exact same reach of river. Comparison of catch rates amongst years indicates that the shad run observed during 2010 was within the same magnitude in prior years. The observed sex ratio in 2010 sampling (0.32 females to every one male) was slightly below the range as observed in previous years (0.38 -1.58 females to males). The shad catch in 1997, was also highly skewed towards males (i.e., 0.38 female to every one male) as observed in 2010; whereas in 1999 the sex ratio favored more females being caught (i.e., 1.5 females to every one male). The catch in 1998 and 2001, were approximately one female to one male being landed. However, no conclusions of annual trends can be inferred due to the limited time duration of available data.

Figure 2. Catch-per-unit-of-effort for American shad in Raubsville electrofishing survey during the historical sampling overlaid with total shad passage at Easton Dam fishway. CPUE is unavailable for 1998 for the Raubsville survey. Monitoring of the 2010 shad passage through the Easton Dam fishway has not yet been completed.

The Smithfield Beach survey represents a long-term egg collection operation by PFBC. Spawning shad are collected via multiple mesh floating gill nets deployed from dusk till approximately midnight. All shad caught are retrieved, stripped of gametes, and mixed to provide fertilized eggs. Collected eggs are then shipped to our van Dyke hatchery where they are hatched and reared for eventual release back into the Lehigh and Schuylkill Rivers. This is the first year that a small, but very valuable portion of fertilized eggs from a restorative standpoint, were given to the Shad In Schools program sponsored by the Delaware River Shad Fisherman’s Association in cooperation with PFBC.

A total of 812 shad, 431 females and 381 males, were collected in 15 nights (May 9 – June 1, 2010) for an overall catch rate of 3.6 shad/net or 0.93 shad/hour. The observed catch rate of shad in 2010 at Smithfield beach is suggestive of a strong returning migration of shad in this pool. Relative to past years, the 2010 shad run was the 4th highest peak in CPUE since 1997. Sizes ranged from 425 - 585 mm total length or 16.7 – 23.0 inches (females: 18.3 – 23.0 inches; males: 16.7 – 22.2 inches) and weight ranged from 695 – 2,244 grams or 1.5 – 4.9 pounds (females: 1.6 – 4.9 pounds; males: 1.5 – 3.1 pounds). Scales and otoliths were also collected for estimation of age, but they have not yet been processed. As with the Raubsville index, catches were initially predominately male, with nearly 100% of catch was males in the first three nights, but by May 16th the nightly catch was dominated by females ranging from 1 to 3 females captured for every one male.

Figure 3. Catch-per-unit-effort for American shad in Smithfield Beach egg take operation.

One interesting observation for the 2010 shad run was the apparent overall smaller size (i.e., total length) of female shad than was typical in prior years. Most of the catch for the 2010 season originated in the smallest mesh nets; whereas in prior years these nets generally only caught males, with the typically bigger females being landed in larger mesh nets. It is perplexing as to why the females tended to be of a small size.

Possibilities could be a combination of gear avoidance, the presence of younger females from the 2007 year class as three year olds, natural mortality of larger females from the spawning population (i.e., the loss of the 2005 female year class as five-year-olds or they simply did not return to spawn this year), poor YOY survival from the June 2006 flood, slow growth of shad in offshore waters, etc. In contrast, the size of males collected at Smithfield Beach from the 2010 run, were more typical as observed in past years.

The apparent shift in the peak size frequency to a smaller size (450 mm TL) group in the 2009 catch was due to the strong contribution (75%) of the 2005 year class as four-year-olds. The influence of the abundant 2005 year class upon catch of males in the 2010 assessment catch appears evident again, considering the dominance of the 475 mm TL size group; however this will need to be confirmed with age determination.

Figure 4. Size distribution of shad collected at Smithfield Beach.

Staff at Area 5 is encouraged by the increased catch of shad at Smithfield Beach, relative to prior years. Historical surveys by the New Jersey Division of Fish and Wildlife, for shad young-of-the-year suggested 2005 and 2007 represented strong production years, which do appear to be influential on the 2010 shad run. Whatever the cause(s) the 2010 shad migration represents an increase from previous years.