Article: Lasley-Rasher RS, Brady DC, Smith BE, Jumars PA (2015) It takes guts to locate elusive crustacean prey. Mar Ecol Prog Ser 538:1-12 doi: 10.3354/meps11481
Following the guts
Highly mobile shrimp species are important members of the marine food web. Not only are they prey for baleen whales and commercial fishes, but they also play an important role in the exchange of nutrients through excretion, foraging (they have the ability to digest detritus), and somatic growth. Their function in the ecosystem is vital and understanding their migration patterns and abundance could help predict the effects of global stressors.
However, sampling these small, mobile crustaceans is difficult. They are too small to be caught in fish trawl nets and they can easily escape the slow plankton net trawls. Fishes, on the other hand, are great at catching and eating these sneaky shrimp, so in order to analyze the shrimp populations, one can first look at their predator’s guts!
Over 300 fishes were caught as part of the Northeast Fisheries Science Center’s (NEFSC) seasonal bottom trawl. Twenty-eight fish species were caught and their stomach contents analyzed (Table 1). Targeting four shrimp families, a statistical program was set up to eliminate factors such as predator encounter rates, predator preference, and number of predators caught and sampled. After all things were set equal, a report of the proportion of fish diets containing the prey species of the four targeted shrimp was made (Fig 1).
What the guts showed, and why we should care
The targeted shrimp species were found to be an important diet component of several bottom feeding predators, specifically the Gadiformes (cod, hake, and Pollack), flounders, and Elasmobranchii (skates and dogfish). It was seen that one of the shrimp species, called mysids, were preferred by juvenile fishes and were not seen in as many adult fish stomachs, but the three other shrimp families were seen in both adult and juvenile fishes. This is surprising, as previous studies have shown mainly juveniles to harvest on the shrimp, and adults to prey on larger crustaceans and small fishes. As fish get larger, they become more equipped to capture and digest larger prey; from an energetic standpoint, catching one larger fish as opposed to several small shrimp is a more effective strategy.
In some fish stomachs, more shrimp were seen in the adults rather than the juveniles, which seems to go in direct opposition with the “catching one larger fish as opposed to several small shrimp” strategy. This occurrence may be due to the opportunistic feeding of certain demersal fish species. They tend to gobble up everything in sight, which could lead them to incidentally ingest crustacean prey while intentionally harvesting on larger fish.
A significant change in the distribution of two shrimp species, the Mysidae and the Crangonidae, was seen when fishes harvesting on these shrimp were more concentrated near shore from February to March (Fig 2). The mean water depth of fishes feeding on these shrimp was significantly lower than the mean water depth of fishes feeding on the other shrimp as well. This occurrence could be due to the migration of the Crangonidae and the Mysidae. A spring bloom occurs in the shallow waters of the Chesapeake and Delaware Bay, making it advantageous for the shrimp to migrate inshore for more resources. If indeed, it is a migration pattern, it is important to understand the exact timing in order for fishes to predict when their juveniles will have the best opportunity to consume the most shrimp.
Crangonidae and Mysidae are unique in that they have the ability to utilize tidal currents to control their horizontal positions, while the other two shrimp species, the Euphausiidae and Pandalidae, are known for their vertical migration. This might be one reason the Euphausiidae and Pandalidae were seen to shift northward in the early spring as opposed to shallow inshore waters like the other two shrimp species.
This study has shown how analyzing the guts of fish can be a useful tool in better understanding the distribution patterns of small crustacean prey families. Utilizing fish diets can be a useful tool in detecting changes in distribution patterns among these important, yet elusive, prey families. Future research can target questions in temporal shifts, prey and predator distributions, and even large scale stressors such as climate change and food-web shift dynamics by following the guts of fishes.