Water is much more difficult to see through than air, and radio waves don't pass through it easily either. That challenges our ability to study many aspects of fish biology and ecology - life history, migration, predator/prey relationships. Even simply counting fish is challenging without the ability to see their movements. Yet it is vitally important to understand fish distribution for both effective environmental protection and a strong fishing sector economy. One of the reasons for boom-bust cycles in fisheries is uncertainty in stock size estimates. Uncertainty can lead to "low-ball" or restrictive allowable catch estimates as a hedge bet, or on the converse, estimates that are too high with the consequence that heavy allowable catches subsequently decimate a fish stock and crash a local economy. The inability to see individual fish also challenges our ability to observe their behavior in un-caged conditions. Historically, this has led to such costly problems as the failure of multi-million dollar construction of fish passageways at dams and the ensuing failure of migrating fish to re-establish breeding populations at upstream spawning grounds.

Fortunately, sound does pass well through water and we can use it in several different ways. We tag fish with acoustic transmitters that encode an identification signal, we view them with increasingly sophisticated sonar imaging (the same solution that dolphins use), and we even listen to the sounds that they produce themselves, which are linked to specific behaviors such as courtship.

In the Navesink River, NJ, we were able to track tagged winter flounder (Fig. 1) close enough to observe spawning. Understanding spawning location preferences and times can help resolve conflicts between dredging needs and protection of the newly laid eggs.

Figure 1. The track of an individual tagged winter flounder (inset) in the upper Navesink River estuary. A location is plotted for up to every 5 sec for 5 days over the course of a month, during both the day and night. For many days it remaind buried, cutting off the tag signal.

Sometimes we use sonar and tags in combination. For ongoing work with collaborators from Woods Hole Oceanographic Institute, I am studying the association of fishes with the Shelf Break Front, an important oceanographic feature extending between Cape Cod, MA and Cape Hatteras, NC (Fig 2). Low frequency sonar is being used to examine sound scattering by fish over large distances, while the structure of the sound-scattering fish schools and their identity is being viewed up close with high frequency sonar and corroborated by acoustic tags. We hope to “mark” a school by marking member fish; the sonar shows us the shape, spacing, orientation, and number of fish in a school while tracking the tagged fish in it verifies the identity and could lead us to an understanding of school formation and breakup. This relates back to producing accurate counts, as fish movements tend to confuse more stratified counting methods.

Figure 2. A school of greater amberjack approach Tom off Cape Hattereas, NC. Their size and schooling behavior produce unique sonar reflections imaged from an AUV. See the Shelfbreak Video for more.

Simultaneous tag mapping and ensonification was already done successfully with Atlantic sturgeon in the Hudson River (Fig. 3, with collaborators from NYDEC and the NEIWPCC) and shortnose sturgeon. in the Delaware River (with collaborators from Environmental Research and Consulting) using an Autonomous Underwater Vehicle (AUV), a robot that carries both the tag detection hydrophone and side scan sonar sonar. I am currently using this approach to study the exposure of Atlantic croaker to hypoxia (low oxygen conditions) along the “Dead Zone” in the northern Gulf of Mexico.

Figure 3. A side scan sonar image of Atlantic sturgeon over rocky bottom in the Hudson River. The image spans 60 meters from right to left. A single 2 m long sturgeon is shown enlarged (inset).

Sonar also allows us to sample in conditions where other collection techniques are severely restricted, such as under the huge piers of New York City.

Here, DIDSON sonar was able to show that intense shading put much of the otherwise available shallows off-limits to the most common mid water fish, but large predators such as striped bass use the mildly-shaded pier edges (Fig. 4) and only nighttime olfactory feeders such as eels can use the vast shaded areas. These fine scale behavior observations can be used by architects to design creative solutions for conflicts between shoreline needs of fish and citizens.

Figure 4. A DIDSON sonogram of striped bass aggregating under the edge of Pier 40 in New York City. The structure on the lower right is a steel pier piling. See the Hudson River study video for more.

About Tom Grothues

Tom grew up in Southern California and maintained a regular contact with the Pacific Ocean throughout childhood and collegiate years at UC Santa Barbara (B.A. in Aquatic Science) and Cal State Northridge (M.S. in Marine Biology). Spearfishing provided an early nucleus around which questions regarding fish distribution and behavior developed, making them relevant on a personal level. A formative series of trips to the Hawaiian Islands with a mentor professor, Howard Craig, helped crystallize a career path – here in the middle of the wide Pacific on geologically young islands were obligate reef dwelling species like butterflyfish that were very unlikely to have swum across deep ocean stretches from distant Indo Pacific reefs. So how did fish disperse there, become established, and why these but not others? Tom worked a four-year stint as the resident marine science technician for the USC research vessels out of Port Los Angeles and serving the Wriggley Marine Science Center at Catalina Island, developing technical experience with the many methods needed to study in the ocean. Tom moved to the east coast in 1994 to pursue a Ph.D in coastal Oceanography at SUNY Stony Brook, further examining the connection that planktonic fish larvae made between populations of adult fish. He currently lives in Tuckerton, NJ near the Rutgers University Marine Field Station where he serves as an Associate Research Professor.