…is the title of a paper just accepted in the Journal of Great Lakes Research, by Duane Chapman et al.* As might be imagined from the importance of the general topic, several stories have popped up in the last couple of days; see here, here, and here for example.
OK don’t panic, not just yet anyway. Still, that title contains two important phrases and one important omission, and unfortunately, that score comes out 2:1, bad to good news.
First the good news. That is contained in the term “grass carp” (Ctenopharyngodon idella), which is to say, thank God it’s not bighead or silver carp (Hypophthalmichthys nobilis and H. molitrix, respectively) recruitment that’s been discovered. The latter are much greater ecological concerns than is the grass carp [# edit: see authors statement on this issue,at bottom]. [The three species are commonly referred to as “Asian carps”, denoting their origin. The bighead and silver carps (“bigheaded carps”) are the species of great concern for the Great Lakes ecosystem, in part because they are perched on the threshold of invasion of Lake Michigan, via the shipping channel connection between it and large, existing populations in the Illinois River].
One might hypothesize that the carp of concern (being in a different genus), might be ecologically different from grass carp in some important way. Such a hypothesis may or may not in fact be correct, because phylogenetic relatedness and ecological similarity are by no means perfectly correlated, but in this case there is indeed a big difference: the latter two are voracious feeders of plankton while grass carp, as the name might imply, are primarily vegetation feeders. This in turn makes for a big difference in competitive effects on other fish, and hence on the structure and function of the entire ecosystem.
Now the bad news. The first is that it’s not just that the fish themselves have been discovered (four fish caught by a commercial fisherman last year), but the evidence of recruitment they provide, i.e. that they very likely originated from some reproducing population in the basin. This conclusion is based on the chemical composition of otoliths (small bones), which were both high and variable in their strontium:calcium ratio, which indicate that the fish were not farm raised (or raised outside of the southwestern Lake Erie area for that matter), because soils in that area have a distinctive Sr:Ca ratio. Furthermore, DNA-based analysis shows that at least two of the four fish caught are diploid, which means they are capable of reproduction (in contrast to triploid fish, which are sterile and not subject to regulation).
Which brings us straight to the second piece of bad news, i.e. that they were discovered in the Lake Erie basin specifically (the Sandusky River). The Lake Erie fishery is vastly more important, both ecologically and economically, than that of any other Great Lake, due to several unique qualities affecting it’s biological productivity. These include relative extremes of: depth (very shallow), latitude (southerly), watershed size to lake volume ratio (very high), and watershed land use/cover (very agricultural, high nutrient inputs). This in turn greatly affects the commercial and sport fisheries, which are both enormous (I tried to find a good economic valuation study for Lake Erie or Great Lakes fisheries, to no avail, but several different lines of evidence indicate the annual total for the Great Lakes fishery to be 7 billion dollars at a bare minimum; see here for some discussion).
As for the recruitment finding, that is important confirmatory evidence supporting a study published last year showing that Lake Erie and its major tributaries had both temperature and hydrologic characteristics judged entirely suitable for spawning for all three Asian carps. It’s one thing to predict suitability, and another thing to actually find evidence that the fish are reproducing there, and if the grass carp are now known to be tolerating the Lake Erie watershed conditions for long enough to reach maturity, this increases the strength of evidence arguing that the bighead and silver carps can do so also.
Modified version of Kocovsky et al., Fig. 1**. Predicted mean summer temperatures for major tributaries to Lake Erie, locations of water intake where lake thermal data were taken, and historic collections of bighead carp and grass carp in western Lake Erie.
As far as anybody knows, the silver and bighead carps are not perched on the threshold of invasion into Lake Erie, like they are into Lake Michigan. However, bighead carp have in fact already been collected from Lake Erie or its tributaries (see the figure), and moreover, the same mechanism (Mississippi to Great Lakes connection), is very possible, via the canal connections in Indiana between the Wabash and Maumee Rivers. And of course, nobody knew any potentially reproducing grass carp were established in the Sandusky River until just now either. And there’s only one way they could have gotten there, by human release, because if they’d come in via existing water connections, they’d almost certainly first have shown up in the Maumee River, not the Sandusky. And if somebody can release grass carp into a Lake Erie trib…well, see the picture and caption at the top of this story for example.
*Duane C. Chapman, Jeremiah J. Davis, Jill A. Jenkins, Patrick M. Kocovsky, Jeffrey G. Miner, John Farver, P. Ryan Jackson, First evidence of grass carp recruitment in the Great Lakes Basin, Journal of Great Lakes Research, Available online 25 October 2013, ISSN 0380-1330, http://dx.doi.org/10.1016
We use aging techniques, ploidy analysis, and otolith microchemistry to assess whether four grass carp Ctenopharyngodon idella captured from the Sandusky River, Ohio were the result of natural reproduction within the Lake Erie Basin. All four fish were of age 1 +. Multiple lines of evidence indicate that these fish were not aquaculture-reared and that they were most likely the result of successful reproduction in the Sandusky River. First, at least two of the fish were diploid; diploid grass carp cannot legally be released in the Great Lakes Basin. Second, strontium:calcium (Sr:Ca) ratios were elevated in all four grass carp from the Sandusky River, with elevated Sr:Ca ratios throughout the otolith transect, compared to grass carp from Missouri and Arkansas ponds. This reflects the high Sr:Ca ratio of the Sandusky River, and indicates that these fish lived in a high-strontium environment throughout their entire lives. Third, Sandusky River fish were higher in Sr:Ca ratio variability than fish from ponds, reflecting the high but spatially and temporally variable strontium concentrations of southwestern Lake Erie tributaries, and not the stable environment of pond aquaculture. Fourth, Sr:Ca ratios in the grass carp from the Sandusky River were lower in their 2011 growth increment (a high water year) than the 2012 growth increment (a low water year), reflecting the observed inverse relationship between discharge and strontium concentration in these rivers. We conclude that these four grass carp captured from the Sandusky River are most likely the result of natural reproduction within the Lake Erie Basin.
** Patrick M. Kocovsky, Duane C. Chapman, James E. McKenna, Thermal and hydrologic suitability of Lake Erie and its major tributaries for spawning of Asian carps, Journal of Great Lakes Research, Volume 38, Issue 1, Pages 159-166, ISSN 0380-1330, http://dx.doi.org/10.1016/j.jglr.2011.11.015.
# I over-simplified this issue, under-valuing the negative impacts of the grass carp themselves, as the authors summarize:
“The implications that grass carp have spawned and recruited in the Great Lakes Basin are profound. The primary effect that grass carp have on a system is removal of submerged aquatic macrophytes. Although few undesirable effects of grass carp have been noted in the Mississippi River Basin where grass carp are established and abundant, those systems are depauperate of submerged macrophytes because of turbidity and highly fluctuating hydrograph. When grass carp are stocked with sufficient density in lentic systems with macrophytes, they can cause nearly complete removal of those plants (Sills, 1970). If populations of grass carp increase in Lake Erie, they may threaten vegetated nearshore areas and wetlands, which are important spawning and recruitment areas for native fishes. Removal of vegetation by grass carp has also been shown to be detrimental to waterfowl (McKnight and Hepp, 1995). Lake Erie coastal wetlands have declined due to anthropogenic effects (Herdendorf, 1992) and further loss of vegetated wetlands would likely result in further loss of the ecosystem benefits that accrue from wetlands, including their high biological productivity, shore erosion protection, water management, nutrient-cycle control, accumulation of sediment, and supply of detritus (Herdendorf, 1987) and mitigation of nonpoint source pollution (Mitsch 1992).”