Atlantic Cod and Eelgrass, oh my!

Well, now I've seen everything. Well maybe not everything, but in all my NY diving, I had never seen this: eelgrass on an exposed, essentially oceanic sandy, rocky bottom, and a school of YOY cod. I have heard about eelgrass in these locations. I have heard that there have been increasing cod landings in NY over the past 2 winters. I have even read that juvenile cod utilize eelgrass. But I had never actually seen it until last week, when we dove along the south-western corner of Fisher's Island.

We were out there for the day looking at some eelgrass for some new projects we are working on in the lab and in addition to collect samples for genetic analysis for a colleague's (Jamie Brisbin's) research. After we were done at our site for the day, we decided to take a quick drop in along the exposed southern shore where the grass was supposed to be extremely tall and growing in a relatively rocky habitat. It was a pretty cool site - I saw typically rocky subtidal macroalgae - kelps, fucoids, coralline - with patchy eelgrass mixed in. It was pretty exciting and cool to see (although my picture below hardly does it justice).


But while I was down there, I was surrounded by what appeared to me to be young of the year (or at the most young juvenile) cod. I am in no means a fish biologist, so I might be off a bit in estimating their age, but they were definitely gadiforms, and I am fairly confident they were Atlantic cod, Gadus morhua. The distinguishing feature for me was the 3 dorsal fins. Either way, I was surprised to be surrounded by this school, although again, these pictures do them no justice. I found it difficult to get good photos - it was late in the day, the water was surgey (I just made up a word I think), and I just couldn't get very close, so I was limited by the capabilities of my Sea and Sea camera. That didn't stop me from trying, mind you. I was swimming, hands extended in front of me and (don't try this at home) holding my breath while diving and snapping away. Everytime I let out a breath, they would swim away. This is poor diving practice, and I wasn't holding my breath for long - just slightly longer than my normal breathing rhythms - it was just my best chance at getting any shots at all.


But then I realized, wow, these are a bunch of young cod and they are staying in this area where there is eelgrass. And I remembered an article I read about YOY cod and survival in eelgrass meadows. And since my experiences with eelgrass have always been in lagoonal-type estuaries where we don't see cod (although we do see their cousins Atlantic tomcod and hake), I was excited to see both eelgrass and cod in the same place (mind you, I had never seen cod while diving either). So I was sitting on the bottom, trying to follow this school of fish and get any good pictures, and thought this is what that paper was talking about. I will detail the paper below.


The basic idea behind the paper by Ann Marie Gorman et al in 2009 was this idea of habitat patch size and edge effects on juvenile cod. I was particularly interested in this paper because the impacts of eelgrass patch morphometrics is something I have spent considerable time working on in regards to bay scallops - my research organism. So any manuscript pertaining to seagrass patch effects I try to read. This paper was pertaining to Atlantic cod, predatory mortality, and edge effects, all things of significance to my research. Since young of the year cod utilize coastal eelgrass habitats as nurseries and predation refuges, varying sizes of patches can have considerable impacts on juvenile survival. The group investigated different size patches, as well as within patch location (along the patch edge, 5 and 10 meters into the patch and into the unvegetated sediment outside the patch), and how those two factors affected the survival of tethered age-0 cod. Obviously, there are all sorts of potential artifacts with tethering mobile individuals in survival studies, however, because they are mobile, there is no other way to look at predatory mortality as specific locations within a given habitat. They observed a relationship which demonstrated lowest survival at intermediate patch sizes and highest survival at the largest patch sizes. And interestingly, they had lowest survival of tethered scallops along the eelgrass patch edge than either within the patch or in the barren habitat - and this survival increased with distance from the edge in both directions. This has been observed in other seagrass habitats, so I bought this. It solidifies the hypothesis that predators in seagrass habitats patrol along the edge of the seagrass, where prey densities are likely to be higher than in unvegetated habitats, and more easily accessible than within the seagrass patch. An interested read for those interested in spatial and landscape ecology, impacts of habitat patchiness on survival, or finifsh predation.

Gorman, A., Gregory, R., & Schneider, D. (2009). Eelgrass patch size and proximity to the patch edge affect predation risk of recently settled age 0 cod (Gadus) Journal of Experimental Marine Biology and Ecology, 371 (1), 1-9 DOI: 10.1016/j.jembe.2008.12.008