|The Pennsylvania Fish and Boat Commission, Fisheries Management Area 5 staff continues to monitor American shad passage through fishways located in the Easton and Chain dams. These dams have historically impeded ascent of shad into the Lehigh River until fishway construction in recent decades.
Area 5 staff are pleased with the 2010 passage through the Easton Dam fishway, which far exceeds the total passage observed in the last two years (2008 – 2009). A total of 1,216 American shad have been documented successfully passing the Easton Dam fishway as of May 25th at 10:50 p.m. Area 5 biologists will continue surveillance of shad passage at both the Easton and Chain dams through the end of June.
Figure 1. Total shad passage through the Easton and Chain Dam fishways since the inception of monitoring in 1994. Note total shad passage for 2010 is provisional as monitoring continues for the remainder of the season.
The 2010 American shad migration into the Lehigh River continues to surpass the low levels posted in 2008 (408 shad) and 2009 (425 shad). Why the difference in numbers passed amongst the 2010 passage and the last two years? While annual natural cyclic fluctuations in shad population abundance strongly influences passage through the Easton Dam fishway; low passage in 2008 and 2009 was likely influenced by adverse climatic and flow conditions that existed during the spring run in those years. Temperatures in 2008 and 2009 fluctuated wildly during the traditional peak periods of the run, with persistent decreases of river temperature, sometimes resulting in a change of a degree or more in a single day. Movement of American shad and spawning activity typically ceases when the river temperatures drops 1°C (1.8°F) or more. In 2010, water temperatures consistently increased following brief cooling periods in the second and third weeks of May, yielding suitable conditions for shad movement.
Flow also plays a part in successful passage of shad through the fishways. Typically, river flows tend to decrease during the shad migration allowing fishway attraction flows to “stand out” from dam spillage, which increases the chance of shad entering the Easton Dam fishway. Historic information tells us that American shad passage through the Easton Dam fishway generally begins when the Lehigh River and Delaware River flows fall below 4,000 cfs and 15,000 cfs, respectively and passage of shad is optimal when flows range between 2,000 to 4,000 cfs and 4,000 to 6,000 cfs respectively. In 2008 and 2009, river flows were constantly changing due rain from cold frontal passage during the entire springtime run, subsuming fishway attraction flows. For the 2010 shad run, river flows generally decreased after a strong influx of rain water during the first weeks of April and May, with minor increases of flow in the second and third week of May corresponding to the passage of weak cold frontal activity. One key difference in river flow amongst the three years was the very strong discharge (~ 9,000 cfs) observed during the first week of April in 2010 that was not evident in either 2008 or 2009 (< 4,100 cfs). This output of water from the Lehigh River may have helped shad differentiate the Lehigh River from Delaware River water based on its unique water chemistry. Thus, allowing Lehigh River shad to more easily “home” to their natal river.
Shad passage through the Chain Dam fishway is another story. Despite greater movement through the Easton Dam fishway in 2010, passage through the Chain Dam fishway continues to remain low. Time-lapsed video recording had only documented a total of 76 shad passing the Chain Dam fishway as of the May 22nd at 10:50 p.m. The efficiency of the Chain Dam shad passage is measured as a percentage of the Easton Dam fishway passage. In previous years, the percent passage at Chain Dam has varied dramatically, including the period after fishway upgrade which occurred in 2000. In the last 10 years, rates varied from a low of 9.5 in 2003 to a high of 44.6 in 2002. However, from a percent passage standpoint, the Chain Dam fishway is performing even poorer in 2010 than in the recent Easton Dam fishway low passage years of 2008 and 2009, of which 20.6 and 14.1 percent of those runs, respectively, successfully passed the Chain Dam fishway. That’s nearly double the passage rate posted so far for the 2010 run (6.2%). One encouraging statistic, however, is that historically total shad passage through the Chain Dam fishway has been highly correlated with shad passage at the Easton Dam fishway. Thus, passage of shad at Chain Dam fishway might increase as the season progresses.
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