Diet research for the shortnose sturgeon

Diet research for the shortnose sturgeon

Craig Springer

It may fall upon the National Fish Hatchery System to prevent the extinction of the shortnose sturgeon (Acipenser brevirostrum). This fish has been listed as an endangered species since 1967. Its recovery plan, published by the National Marine Fisheries Service in 1998, calls for a captive-rearing program since wild populations may need to be augmented with hatchery-raised fish. Since the late 1980s, national fish hatcheries, the National Marine Fisheries Service, and state agencies have investigated fish culture techniques to raise this rare and unusual fish.

The Bears Bluff National Fish Hatchery in South Carolina, a satellite of the Warm Springs Regional Fisheries Center in Warm Springs, Georgia, is studying the early life history of this ancient fish. As part of this effort, it has conducted diet studies in captivity to increase the efficiency of hatchery propagation. Ultimately, that will translate into improved survival of young fish, benefiting their conservation and achieving it at a lower cost.

The shortnose sturgeon has suffered from the typical suite of environmental impacts that have hurt other fishes, says Bears Bluff fish biologist James Henne. For example, shortnose sturgeon naturally made extensive upstream migrations to reproduce, but dams have impeded their access to historic spawning habitat. Changes in river flows from dams have also affected the fish by altering oxygen levels, river flows, and water temperatures. Over-fishing and incidental catches in commercial fisheries operations have had impacts on the shortnose sturgeon as well.

The fish is named for its snout, which sets it apart from all the other sturgeons. It’s most similar to the widely distributed lake sturgeon that naturally occurred over much of the inland waters of the South and Midwest and through the Great Lakes. The shortnose sturgeon was naturally confined to the estuaries and major streams tributary to the Atlantic coast from Florida to New Brunswick. Across its range along the coast, it does appear locally abundant in southern rivers like the Santee and Altamaha, but culture programs and intensive protection are still needed to conserve the species as a whole.

Toward that end, the Bears Bluff NFH maintains a refugium population of adult shortnose sturgeon that originated from hatchery stock. Over the last 10 years, hatchery biologists have made impressive progress in learning sturgeon culture techniques.

“Early on, the experience was simply learning to keep fish alive. There’s a steep learning curve when you’re trying to culture a fish where the techniques aren’t well known,” said Henne. “We’ve gotten better. Though our fish aren’t intended for release in the wild, in the event their conservation necessitates that, we have the technology and techniques to do it.”

That readiness comes in part with the advances made in early-life diet. Hatchery biologists at Bears Bluff fine-tuned feeding techniques to get through what amounts to a bottleneck: converting larval fish from a natural to a prepared diet. Mortality can be extremely high at that stage of the life cycle. The work can be labor-intensive, requiring manual feeding of brine shrimp about every four hours over the 24-hour period for up to 60 days. Most feeding is done at night, since the animal is naturally most active nocturnally, and that requires much staff time at odd hours.

Hatchery biologists tested six different feeding regimes, including an automated regime using formulated feed mixed with live feed. After a 30-day trial, the automated feed regime proved best. At the end of the trial period, the larval fish fed a mix of natural food and formulated food had an increase in body weight 46 percent higher than larval fish fed the traditional live-feed only. This regime reduces labor costs as well as benefiting the fish.

Not only does this work move things along for shortnose sturgeon, but the findings are probably applicable to other sturgeon elsewhere. Since the findings of this research were published in the American Fisheries Society’s North American Journal of Aquaculture, Henne has corresponded with researchers from across the United States, Spain, Germany, and Portugal.

The work at Bears Bluff NFH doesn’t stop with fish culture. As the larval fish become juveniles, hatchery biologists use them to advance the understanding of what can be done to conserve sturgeon in the wild. Marking fish for release in the wild is important for estimating populations or simply recognizing during future surveys whether a fish is from hatchery stock or a product of natural reproduction. Henne and others have tested implanting PIT (passive integrated transponder) tags in shortnose sturgeon to identify the minimum size at which a young fish can be tagged and the best place on the body to place the tags for the long-term.

Bears Bluff biologists have learned that a suitable place to tag small fish may be under the third cranial scute, a large bony plate. After months of study, survival and growth do not appear to be influenced by tagging. The tags thus far are well retained. In the end, Henne says if the growth and retention rates follow the current trend, the study at Bears Bluff should lend confidence to management biologists monitoring and studying populations in the wild.

Ultimately, conservation of the shortnose sturgeon depends on continued cooperation among the vested partners, not the least of which is the ongoing commitment of biologists at the Bears Bluff National Fish Hatchery.

COPYRIGHT 2007 U.S. Fish & Wildlife Service

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