In April 2013, Entergy published its most recent annual Marine Ecology Study (Report No. 81a and 81b; covers 2012) in which it reports on entrainment and impingement of marine organisms. Forty species were affected by entrainment and impingement in 2012. Unusually high entrainment densities occurred on 85 occasions and involved 16 species. Lobsters were entrained at a much higher rate in 2012 compared to previous years. Dominant species varied by season, but species commonly found in entrainment samples included American plaice, windowpane/yellowtail/four-spot flounder, Atlantic cod, fourbeard rockling, Atlantic mackerel, sand lance, grubby, rock gunnel, tautog, cunner, labrid, hake, butterfish, black sea bass, silver hake, Atlantic herring, among others.

Energy’s report estimates an annual impingement total of 9,287 fish. This is lower than the long-term annual mean of 46,341 fish per year. Atlantic silvesides, blueback herring, Atlantic menhaden, butterfish, alewife, Atlantic tomcod, grubby, red hake, winter flounder, black seabass, northern puffer were the most commonly impinged fish in 2012, accounting for 85% of the takes. The most common invertebrates impinged in 2012 included sevenspine bay shrimp, ribbon worms and longfin squid. Entergy’s report estimates an annual impingement total of 11,931 invertebrates.

Here’s a small sample of numbers of marine organisms impinged/entrained in 2012 by Pilgrim. Keep in mind that estimates for many of the most commonly entrained species were not reported in detail in Entergy’s report, therefore are not included below. Only a select number of species are detailed in the report.

Species Number Eggs/Larvae Entrained Number Impinged
winter flounder 13,809,641 277
cunner 1,639,358,542 166
Atlantic mackerel 43,600,940 18
Atlantic menhaden 3,542,315 1,469
Atlantic herring 3,882,289 (larvae only) 68
Atlantic silverside Larvae only; but number not reported 1,951
River herring Number not reported 2,218
Atlantic cod 3,949,254 99
Lobster 2,882,660 (larvae only, includes impingement and entrainment combined)


A major problem with determining the effects of entrainment on fish and invertebrates is the inability to accurately quantify the number of eggs or planktonic larvae being entrained, as well as the inability to correctly determine actual ecological impacts. Related to the latter problem, Entergy uses models to determine adult equivalency, or EA. The models essentially try to determine the number of adults that would have been produced by the entrained larvae/eggs.  Entergy uses several methods to determine EA and then averages the results to produce EA estimates. The problem is that these models have many assumptions and are unlikely to yield accurate results.

Here’s an example from Entergy’s report:  Although 2,882,660 American lobster larvae were entrained/impinged in 2012, using EA methods this reportedly only equals 94 adult equivalents taken out of the population!

A report done by Stratus Consulting in 2002[1] estimated the average number of fish that were killed annually through entrainment and impingement at Pilgrim to be 14.5 million. But the numbers in Entergy’s 2012 report are much lower. Entergy’s report doesn’t actually provide a total average number of fish killed each year. A detailed review was only provided for seven species [six fish and American lobster], despite the fact that 40 species were found in entrainment samples.  Furthermore, the majority of the most commonly entrained species were not included in any detailed review. Based on the numbers provided for those seven species, it seems that Entergy’s estimate for total fish killed in 2012 through impingement and entrainment would be in the hundreds of thousands (much less than the estimate provided by the Stratus Consulting report) – but it’s impossible to estimate without detailed data for all 40 species found in entrainment samples.

This is one of the many problems associated with a once-through cooling system – it’s nearly impossible to accurately estimate the impacts of entrainment and impingement at the population-level, and especially to the ecosystem as a whole. The state of California, which has many power plants along its coast, has researched these problems at length[2]. The result? A new statewide policy that phases out destructive once-though cooling systems due to the environmental damage these systems cause – entrainment and impingement included.

Next month, Entergy’s semi-annual report should be available for the first half of 2013. We will keep you posted on the numbers of marine organisms entrained and impinged so far in 2013, as reported by Entergy.

[1] Stratus Consulting. 2002. Habitat-based replacement costs: An ecological valuation of the benefits of minimizing impingement and entrainment at the cooling water intake structure of the Pilgrim Power Generating Station in Plymouth, Massachusetts. Report for the U.S. EPA, Region 1