Stomach Contents

Saltwater Marine Fishing Regulations Rhode Island Saltwater Fishing

What are fish eating when not stealing your bait?

By Thomas Angell, Principal Marine Biologist, RI DEM Division of Marine Fisheries

When you have caught one of the several marine fish species that reside in or regularly visit Narragansett Bay and our coastal waters, do you ever wonder what it was eating other than the bait you used, how old it was, or if it was mature? When cleaning your catch, have you ever opened the fish’s stomach to see what was there or looked at a fish scale to get an idea of how old it was? Knowing these things may just be interesting bits of trivial information for a recreational angler but are very important for stock assessment biologists and fishery managers in determining the status of our fishery resources. RIDMF staff biologists Nicole Costa, Christine Denisevich and Thomas Angell have been collecting and analyzing this data to provide for stock assessment purposes.

Age and growth information is essential in estimating the age-structure of a fish population. Understanding the age-structure of a population allows scientists to make informed management decisions regarding acceptable harvest levels for a species. In recent years, diet composition of finfish has become increasingly important in understanding the age and growth of a population. Diet composition of a species may help to inform managers on whether an observed change in a population may be due to prey availability. Understanding predator-prey dynamics can also allow managers to utilize a multi-species modeling approach by which they can better understand not only the population dynamics of a target fish species, but other choke or prey species that may be associated with the target species.

Since 1987, the RI Division of Marine Fisheries (RIDMF; formerly part of the Division of Fish and Wildlife) has collected age and growth data for several fish species important to both recreational and commercial fishing interests in RI, including black sea bass, bluefish, menhaden (pogies), scup, striped bass, summer flounder (fluke), tautog, weakfish; winter flounder was added to the sampling more recently. Samples are obtained from various RIDMF sources including the coastal trawl surveys, finfish ventless pot survey, and Narragansett Bay juvenile finfish survey, as well as from commercial fish trap or gillnet operations, finfish dealers, and donations from recreational party/charter boat businesses and individual fishermen. Anatomical structures collected and used to determine age include scales, ear bones (otoliths), gill covers (opercula), and the first pelvic fin spine. Size (fork length or total length) and weight (if whole fish) are also collected. Collection of stomach content, sex, and maturity stage data for the species listed above was initiated in 2014, with a minimum of 40 stomachs examined annually for each fish species. Identification of stomach contents is made to the lowest taxonomic level possible. This study is designed to characterize the age-structure and diet composition of these fish stocks and will supplement data collected in the Northeast Fisheries Science Center (NEFSC) spring and fall surveys as well as the Northeast Area Monitoring and Assessment Program (NEAMAP), which do not sample within Narragansett Bay. Data collected in this study is already used in several stock assessments, and that number is expected to increase each year as benchmark stock assessments are conducted and ecosystem-based modeling approaches are further developed. Additionally, this study satisfies the requirements of ASMFC Fishery Management Plans (FMP’s) for tautog, bluefish, menhaden and weakfish which require the state of RI to collect a minimum number of age and growth samples annually for stock assessment purposes. Table 1 summarizes the type and number of age structures collected, number of stomachs examined, and number of sexual maturity determinations made for each fish species during 2014-2019. Figure 1 summarizes the proportional contribution of prey items for 8 important fish species. Figure 2 shows annual growth marks of an otolith (ear bone) and scale from a striped bass. Given the amount of growth past the second annulus and the sample date (4/27/17), the fish had not laid down it’s annulus for the year and would be assigned an age of 3 years.








Black sea bass























Striped bass



83 (dorsal)



Summer flounder









214 (anal), 130(pelvic)








Winter flounder






Table 1

Analysis of 2014-2019 stomach content data for each fish species with “unidentifiable” contents removed from the analysis resulted in the following (see Figure 1):

Figure 1 – Proportional contribution of prey items by fish species

Black sea bass (BSB) – stomach contents dominated by crustaceans (43%; rock crab, Jonah crab, lobster, amphipods, mud crabs, mantis shrimp, hermit crabs, mud shrimp, Asian shore crab, sand shrimp, spider crab), cephalopod molluscs (26%; squid, snails), followed by finfish (14%; sand lance, clupeids, black sea bass, butterfish, scup) and bivalve molluscs (13%; blue mussel, soft-shell clam, razor clam); minor contributions came from gastropod molluscs (1.4%), with all “other” identifiable contents combined (algae, aquatic plants, cnidaria, platyhelminths, polychaetes, sand/rocks, sipunculids, and urochordates) accounting for 2%.

Bluefish (BLU) – stomach contents dominated by finfish (92%; menhaden, scup, butterfish, sea robin, bay anchovy, silversides) and cephalopod molluscs (8%; longfin squid). All “other” identifiable contents combined (algae, aquatic plants, sand/rocks) accounted for 0.013%.

Menhaden – Due to the fact that menhaden are filter feeders, all stomach contents encountered during this study were liquefied, with prey item(s) unable to be identified and classified. Due to this, no menhaden stomachs have been examined since 2017 and are not included in Figure 1. Generally, menhaden stomach contents will reflect the dominant planktonic species present at the time of sample collection.

Scup (SCU) – stomach contents dominated by polychaetes (32%; bamboo worms, blood worms, clam worms, red-lined worms), bivalve molluscs (24%; razor clam, blue mussel, softshell clam), and crustaceans (20%; amphipods, mud crab, hermit crab, sand shrimp), followed by cephalopod molluscs (10%; longfin squid) and finfish (9%; menhaden, bay anchovy), with a minor contribution from gastropod molluscs (3%; slipper shell, dog whelk). Algae, bryozoa, cnidaria, sand/rocks, and sipunculids made up the “other” identifiable contents (2%).

Striped bass (STB) – stomach contents dominated by finfish (91%; menhaden, Atlantic herring, scup, butterfish, sand lance, Atlantic mackerel, black sea bass, silver hake), followed equally by crustaceans (4%; mantis shrimp, sand shrimp, amphipods), cephalopod molluscs (4%; longfin squid). Algae, aquatic plants, bivalve and gastropod molluscs, polychaetes, and sand/rocks made up the “other” identifiable stomach contents (1%).

Summer flounder (SFL) – stomach contents dominated by finfish (56%; menhaden, scup, weakfish, sand lance, Atlantic herring, black sea bass, bay anchovy), cephalopod molluscs (23%; longfin squid), and crustaceans (20%; mantis shrimp, sand shrimp, amphipods), with bivalve molluscs, gastropod molluscs, nematodes, and sand/rocks making up the “other” identifiable stomach contents (0.4%).

Tautog (TAU) – stomach contents dominated by crustaceans (60%; rock crab, green crab, mud crab, spider crab, Asian shore crab, lobster), followed by bivalve molluscs (20%; blue mussel, blood ark, bay scallop) and gastropod molluscs (13%; slipper shell, greedy dovesnail, chink shell, dog whelk), with minor contributions from maxillopods (4%; barnacles) and algae (2%). Bryozoa, polychaetes, and sand/rocks made up the “other” identifiable stomach contents (1%).

Weakfish (WEAK) – stomach contents dominated by finfish (91%; butterfish, menhaden/clupeids, scup, silversides), followed by cephalopod molluscs (6%; longfin squid) and a minor amount of crustaceans (2%; shore shrimp, sand shrimp, sand crab). Algae and polychaetes made up the “other” identifiable stomach contents (0.16%).

Winter flounder (WFL) – stomach contents dominated by polychaetes (40%; brush worm, blood worm, red-lined worm, bamboo worm, opal worm) and cnidarians (35%; anemone, hydrozoa), followed by crustaceans (15%; amphipods, sand shrimp, hermit crab, mud crab). Minor contributions came from algae (4%; barrel weed, sea lettuce), gastropod molluscs (2%), nemerteans (<2%), and bivalve molluscs (<2%). Bryozoa, finfish, sipunculids, and sand/rocks made up the “other” identifiable stomach contents (1%).

Sectioned otolith (a) and scale impression (b) from a striped bass with 2 annuli marked and growth beyond the last annulus. Given the amount of growth past the 2nd annulus and the date of capture (4/25/17), the fish has not yet laid down it’s annulus for the year and would therefore be assigned an age of 3.