Diet Analysis

Lab Processing

Species Priority List

The ChesMMAP survey processes stomachs from every species caught in order to obtain a broad picture of the diet composition of individuals in the Bay.

During the design phase of the NEAMAP Survey, the Operations Committee developed a set of species priority lists to identify and rank those of management interest and, in turn, guide the collection of biological data (see table below). The NEAMAP survey's broad geographic and temporal scale presents a unique opportunity to document and quantify diet habits for a broad variety of species at several trophic levels.

Diet Composition

It is well known that fishes distribute in temporally and spatially varying aggregations. The biological and ecological characteristics of a particular fish species collected by fishery-independent or -dependent activities inevitably reflect this underlying spatio-temporal structure. Intuitively, it follows then that the diets (and other biological parameters) of individuals captured by a single gear deployment (e.g., ChesMMAP or NEAMAP tow) will be more similar to one another than to the diets of individuals captured at a different time or location (Bogstad et al. 1995).

Under this assumption, the diet index percent by weight for a given species can be represented as a cluster sampling estimator since, as implied above, trawl collections essentially yield a cluster (or clusters if multiple size groups are sampled) of the species at each sampling site. The equation is given by (Bogstad et al. 1995, Buckel et al. 1999):

where:

and where n is the total number of clusters collected of the fish species of interest, Mi is the number of that species collected in cluster i, wi is the total weight of all prey items encountered in the stomachs of the fish collected and processed from cluster i, and wik is the total weight of prey type k in these stomachs. This estimator was used to calculate the diet compositions of the NEAMAP Priority 'A' species (for those where diet data are currently available); the resulting diet descriptions are included in this report. Again, while these diets reflect a combination of data collected from the six fullscale survey cruises (fall 2010 data are not yet available), presentations of diet by sub-area, year, cruise, size, age, etc., are possible.

The percent weight (%W), percent number (%N), and percent frequency of occurrence (%F) indices are all useful in different contexts so each is presented here. For %W and %N, only those specific prey types that reach a 1% threshold in the overall diet are shown individually. All others are summed into broader taxonomic categories. Further, for these indices, closely related prey types (e.g. different species of mysids or of amphipods) are generally summed and reported together as a group. For %F, only prey types that reached a 2% threshold in the overall diet are shown individually. It must be noted that for %F, prey types are not additive because each predator sample may be counted multiple times if multiple prey types were consumed. Thus overall percentages for broad taxonomic categories (e.g. fish, molluscs, etc.) is not equal to the
sum of its constituents. Also, the sample sizes reported under %F are larger than for %W and %N because empty stomachs are counted in the former but not for the latter two. Finally, it is worth noting that the %N and %F indices are calculated using the cluster sampling estimator as
well, following the same form given in Equation 3 and Equation 4.

References:

Hyslop, E. J.  1980.  Stomach content analysis--a review of methods and their application.  Journal of Fish Biology.  17:  411-429.

Bogstad, B., M. Pennington, and J. H. Volstad.  1995.  Cost-efficient survey designs for estimating food consumption by fish.  Fisheries Research.  23(1-2):  37-46.

Buckel, J. A., M. J. Fogarty, and D. O. Conover.  1999.  Foraging habits of bluefish, Pomatomus saltatrix, on the U.S. East Coast continental shelf.  Fishery Bulletin.  97(4):  758-775.

Diet Analysis Laboratory Supplies

• Trays for processing stomachs typically include the following items:                 • Calipers                 • Forceps (2) - straight & curvy                 • Plastic Petri Dishes                 • Scissors                 • Spoonula                 • Watch Glass

• Stomach Samples 

• Dissecting Microscope

• Analytical Balance - minimum of 0.001 g to a maximum of 410 g

• Gloves

• Laboratory Squirt Bottle

• Data Sheets (or FEED program for direct entry)

• Trash Bin

• Plastic Cups

Diet / ID Guides

• ChesMMAP Diet Guide
• NEAMAP Diet Guide
• Fishes ID Guide
• Invertebrate ID Guide

Food Webs

ChesMMAP Diet Analysis - Food Web Hive Plots

The following list is a guide to some of the technical terms applied to invertebrate prey types encountered in fish stomachs, but it is not an exhaustive list by any means.

Amphipods

Usually no bigger than a fingertip, amphipods are small crustaceans that form a very important prey base for our small and juvenile fishes.  Most of the amphipods we identify are laterally flattened, like the common burrower amphipod and four-eyed amphipod.  Some are dorso-ventrally flattened, like Corophium spp.  Also classified as amphipods are the unique skeleton shrimp, commonly found in stomachs of structure- or seagrass-associated predators.

Bivalves

Clams, mussels, scallops, and oysters belong to the taxonomic class, bivalves.  We frequently find clam parts, whether they are broken shell pieces or the soft foot or siphon, in the stomachs of adult bottom feeders such as drums, skates, and stingrays.  We seldomly find whole bivalves, so identification to species is rarely successful.  Some common bivalve prey species that are distinctive enough to positively identify are the common razor clam, stout tagelus, and ark shells.

Bryozoans

Bryozoans, sometimes called moss animals, are tiny individuals that form colonies.  The colonies encrust submerged surfaces.  When we find bryozoans in fish stomachs it is unclear whether the bryozoans are incidentally ingested by the predator or being targeted as prey.  Massive colonies of the bryozoan we call dead man's fingers are frequently encountered in the trawl net during our monitoring cruises.  We categorize these colonies as habitat, so perhaps it's safe to assume a predator was foraging in the habitat targeting more definitive prey items and ingested the bryozoan incidentally. 

Cephalopods

Squid belong to the taxonomic class, cephalopods, which also includes the octopus and nautilus.  Although we identified only one octopus as a prey item, squid are commonly eaten by large piscivores such as summer flounder, bluefish, dogfish, and clearnose skate.  The longfin inshore squid is not only an important prey type for these predators, but it is also an important commercial fishery resource.

Cnidarians

The most popular cnidarians are jellyfish and comb jellies.  Although these taxa are difficult to significantly quantify in our diet analyses, we have some evidence of jellyfish being eaten by butterfish, harvestfish, and Atlantic spadefish.  Cnidarians also include sea anemones and hydroids, which we occasionally identify in winter flounder diets and even less frequently in a few other species.  Massive colonies of hydroids are frequently encountered in the trawl net during our monitoring cruises.  We categorize these colonies as habitat, so perhaps it's safe to assume a predator was foraging in the habitat targeting more definitive prey items and ingested the hydriod incidentally. 

Copepods

Calanoid copepods are an important prey type for small and juvenile fishes, especially in the shad and herring family.  These crustaceans are tiny, most less than 1 mm, but very abundant as zooplankton in the water column.

Decapods

Decapods encompass the very large group of crabs, shrimps, and lobsters.  These are some of the most important prey types for a diverse array of predator species of various life history stages.  The most frequent decapod we encounter in fish stomachs is sand shrimp, a small shrimp whose presence is nearly ubiquitous in the diets of fishes of all sizes.  Other common shrimp we encounter are the long-eyed estuarine shrimp, bristled longbeak, roughneck shrimp, and short-browed mud shrimp.  Mud crabs are consumed by Chesapeake Bay species, while fishes in our nearshore Atlantic habitat consume Atlantic rock crabs.  Hermit crabs and lady crabs are commonly eaten by fishes in both habitats.  Decapods are even consumed as larvae, called zoea or megalopae, in the zooplankton along with copepods, by the smallest fishes. 

Echinoderms

Echinoderms include sea cucumbers, sea urchins, sand dollars, sea stars, and brittle stars.  So far, we have only seen brittle stars and sand dollars in significant quantities, and even those were relatively rare.  All of the brittle stars and sand dollars we identified were broken up in pieces in the stomachs primarily of scup, Atlantic croaker, and Northern puffer. 

Gastropods

Many common names have been given to these animals, but for us gastropod refers mainly to snails.  Unfortunately the majority of snails we encounter in fish stomachs are too small or crushed to identify to the species level.  However, the barrel bubble is distinct and common enough that we know it when we see it.  Often the shape or size of the snail's operculum, or the hard covering over the opening of the shell, that allow us to identify the moon snail or New England dog whelk, both of which we find relatively frequently.   

Isopods

Isopods, like amphipods, are small crustaceans that resemble the pill bug you see on land, but are obscure enough that many do not have common names.  We find them mainly in the diets of smaller fishes.  They are generally larger than amphipods and are always dorso-ventrally flattened.  Isopods differ in appearance between the species enough that they usually can be positively identified.  The most commonly identified isopod in our nearshore Atlantic stomach samples is Politolana concharum.  In the Chesapeake Bay, interestingly, the most common isopod identified is a  parasitic species, Livoneca redmanii.  The mound-back isopod, slender isopod, Chiridotea spp., and Sphaeroma quadridentatum are also common prey items.

Miscellaneous

Anything we cannot identify as animal or plant, or cannot differentiate as animal or plant from other unidentified material in the stomach contents is classified as miscellaneous.  Included in the miscellaneous category are any animal or plant parts that are too digested to identify as such; mineral material like rocks, sand, or mud; other organic material or detritus; animal tubes; and artificial material such as trash or fishing lures.  Artificial material is relatively rare, but always interesting to find.

Mysids

In Chesapeake Bay food webs, mysids are one of the most important prey types for smaller fishes using the Bay as foraging habitat prior to reproductive age.  Mysids are small crustaceans (<1 cm), sometimes called "oppossum shrimp", although they are not shrimp.  A key feature in differentiating a mysid from a small shrimp is the presence of statocysts, a pair of tiny bubble-like structures embedded in the tail.  The shape of the tail can help identify the mysid to species level.  The most common mysid we find is Neomysis americana, followed by Americamysis bigelowi.  Sometimes we find these two species together in a sample, and in great numbers, so we cannot reasonably identify them all to species level.

Other Crustaceans

Arthropods are taxa that include crustaceans, but also insects, spiders, and horseshoe crabs.  The taxonomic grouping of crustaceans is very broad and has many subgroups, including decapods, amphipods, isopods, mysids, copepods, and stomatopods.  All other crustaceans we find are infrequent enough that we can group them together into one category. Like copepods, most are zooplankters:  cumaceans, cladocerans, tanaids, and ostracods.  We also include barnacles, which have a planktonic larval stage. 

Plants

Any type of vegetation we find in stomach samples is grouped into the plant category.  We can identify sea grasses, seaweed or macro algae, leaves, and seeds if they are in good shape, but most often the vegetation remains unidentied or identified as detritus.  It is unknown whether our fishes deliberately consume the vegetation for nutrition, but it seems more likely the plants are ingested with animal prey using it as habitat.  Plant material identified in our fish diets is relatively uncommon, but we have quantified some in winter flounder, winter skate, smooth dogfish, and Atlantic croaker.

Stomatopods

Stomatopods are an important prey type for larger age class predators in both the Chesapeake Bay and nearshore Atlantic habitats.  Stomatopods, also called "mantis shrimp", are crustaceans, but are not true shrimp, which are decapods.  Stomatopods are quite distinct from shrimp in that they are dorso-ventrally flattened and have distinct head, body, and tail regions.  While we do find stomatopod larvae, and smaller juvenile specimens, we encounter adult forms greater than 8-10 cm most frequently.  In Chesapeake Bay, we identify exclusively Squilla empusa, and in the nearshore Atlantic, we have found Nannosquilla grayi, and Platysquilloides enodis.

Tunicates

Otherwise known as sea squirts, tunicates are colonies of indivuals whose size varies from species to species.  Although tunicates are relatively rare in our fish diets, the genus we most frequently encounter is Molgula, which has large and globular individuals.  Rarely, we will encounter a stomach containing eelgrass with a tunicate colony encrusting it.  When we find Molgula in fish stomachs it is unclear whether they are incidentally ingested by the predator or being targeted as prey.  Massive colonies of Molgula are frequently encountered in the trawl net during our monitoring cruises.  We categorize these colonies as habitat, so perhaps it's safe to assume a predator was foraging in the habitat targeting more definitive prey items and ingested the tunicate incidentally. 

Worms

Worms are an important soft-bodied prey source for small and large bottom feeders like Atlantic croaker, spot, winter and little skates, and winter flounder; but large worms are also consumed by voracious predators like striped bass.  The vast majority of worms we identify are polychaetes, or segmented worms; and the vast majority of polychaetes, broken down as they are, are not reasonably identifiable to a lower taxon.  However, common taxa we have been able to identify are Glycera, Pherusa affinis, Arabella iricolor, Ophelia denticulata, Travisia carnea, Nereis, Pectinaria gouldi, and Clymenella torquata.  Our black sea bass sometimes eat Aphrodita hastata, or sea mouse, an atypical and bizarre-looking polychate.  Our infrequent non-segmented worm encounters include sipunculids (peanut worms), nematodes (roundworms), acanthocephalans (spiny-headed worms), and echiurans (burrow worms).  Regarding soft-bodied worms, it is important to note that when performing diet analysis using direct observation of stomach contents, these prey sources are very likely underrepresented.  Because most likely the worms are broken apart and digested more quickly than prey types with a hard shell or scales, we may hypothesize that fishes consume more worms than we can observe and quantify in the stomach contents.