OCEANS: So where have all our fish gone anyway? Think small!


 

 

“If there is nothing for fish to eat, there are no fish for you to eat!”

Color photo of zooplankton Mysis relicta.

Mysis 

If you spend much time exploring the intertidal waters of Cobscook Bay, you are likely to have seen a variety of creatures that look like shrimp. Many shrimp-like invertebrates found here may not have any direct economic importance, but are important as food for larger marine animals and for the role they play in processing waste and plankton. One such creature, the mysid shrimp, appears to be an important winter food for young groundfish.

Because mysids carry their young in a pouch beneath their belly, they have been given the common name of opossum shrimp. Four species of mysid shrimp are recorded from the nearshore waters of Cobscook Bay. Each have slightly different habits. The focus of this article will be on one particular species, with the scientific name Mysis stenolepsis, which appears to be most significant as a food for wintering groundfish.

Mysid shrimp are not really shrimp at all, but look very similar. Smaller in size than the true shrimp and with a distinct set of antennae, mysids can be easily confused with krill, another shrimp-like creature; however, they inhabit a different space in the water column. At most times of the year, mysid shrimp are found in shallow, intertidal waters with mud bottoms or granular sediments and seaweeds. Krill are found in deeper waters, from the surface down to depths of about 275 meters (or roughly 900 feet).

Biologist, Art MacKay made interesting observations about mysid shrimp during 30 years of diving and inventorying intertidal sites within Passamaquoddy Bay as a consultant for research institutions.

MacKay says, “In the 1970’s, I worked on a resource inventory of inshore areas of Passamaquoddy Bay for five years. I found that mysids were extremely abundant in intertidal waters in Oak Bay and other highly productive areas between St. John and Moncton. A single hand seine would be full of pounds of mysid shrimp, with sand shrimp, sticklebacks, and smelt mixed in.”

He adds, “We could collect them up to December, and then with a drop in water temperature they would be gone. I was curious about where the mysids went. At one point, we found a ball of them in deeper water out near the St. Andrews wharf.”

While looking up research on fish, MacKay found a study of groundfish in Passamaquoddy Bay that showed several groundfish species, including small pollock and cod, shifted from krill to eating mysid shrimp as a primary food during the winter.

“There used to be overwintering populations of groundfish at the mouths of all the major estuaries in Passamaquoddy Bay. When mysids disappeared from the shallow waters, the timing was almost perfect for when the groundfish were found to have made the switch to eating mysid shrimp. The mysids dropped into deeper water in large quantities as the temperatures fell, and provided food for these overwintering fish,” says MacKay.

“In recent years I’ve done some more work in Oak Bay and discovered that the populations of mysid shrimp were decimated. They were no longer at the sites where I once found them in abundance. Something in that estuary has affected the production. It looks the same superficially, because sand shrimp have taken over, but the mysids have gone.”

MacKay notes an increase in development along the shores may have played a part in the disappearance of mysid shrimp. “It appears to me that nutrient inputs into estuaries from septic systems have exceeded the outputs, and this has changed the chemistry enough to make it unsuitable for mysids. The water quality has diminished to the point where mysids were the first to go. We have also seen the near shore fishery pushed away from shore, the herring fishery is gone, and many overwintering populations of groundfish have disappeared.”

At a recent visit to Pottle Stream, which empties into Lewis Cove in Perry, MacKay was thrilled to see mysid shrimp in abundance. MacKay says, “Cobscook Bay appears to be doing better because greater mixing occurs in the water there, although the more intense aquaculture activity in Cobscook may add nutrients to the system.”

The relationship between mysid shrimp and wintering groundfish also appears to hold true for areas west of Cobscook Bay. While studying sand shrimp and groundfish in near shore areas from Lamoine to York, Mike Brown, a marine resource scientist for the Maine Department of Marine Resources, found that the red hake, codfish, and winter flounder he was examining had belly’s heavy with mysid shrimp. Fortunately, fish don’t chew, so when the stomach content is examined, it isn’t too hard to figure out what the fish have eaten.

Brown states, “We were catching fish 1 1/2 inches to 4 inches in size, from November right through January, to see if they were feeding on sand shrimp. Instead, we found the stomach contents were mostly comprised of mysids.”

The mysids they were catching were reproductive females. “Since one of the sand shrimp’s favorite prey is mysids, the sand shrimp were probably inshore to feed on the brooding females, while the young groundfish fed on both sand shrimp and mysids. There’s no question that mysids are important food for nearshore groundfish.”

We can assume that the importance of mysids to small groundfish holds true in Cobscook Bay as well, but figuring out scientifically whether or not there is a connection between mysid shrimp and groundfish here is no easy task. Mysid shrimp are not on a list of research priorities, partly because of the difficulty in studying them. And, although DMR is conducting a groundfish survey of inshore areas off the coast, Cobscook Bay is not included, so it’s difficult to say where young groundfish are being found. Understanding what groundfish eat at various stages in their life cycle in Cobscook Bay would be an important step in figuring out how to recover groundfish populations here.

This column was prepared by Cheryl Daigle. Cobscook Soundings is a monthly column produced by the Maine Chapter of The Nature Conservancy. It seeks to share what is known about the workings of the Cobscook Bay marine environment, so that all who make decisions about the use or care of the bay have the best available information.  

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