VII. FOOD HABITS OF THE MUMMICHOG (Fundulus heteroclitus)
Frank W. Steimle, Jr.
Postal Addresses: National Marine Fisheries Serv., 74 Magruder Rd., Highlands, NJ 07732
E-Mail Address: Frank.Steimle@noaa.gov
INTRODUCTION
The intertidal salt marsh is used as a source of food or feeding grounds for a number of estuarine organisms, including fish and invertebrates. Some of these organisms are in turn prey for larger organisms. The mummichog, or common killifish, Fundulus heteroclitus, lives along the saltmarsh fringe and is generally considered an important link in the estuarine food web supporting valued biological resources. The mummichog uses the tidally flooded marsh to access S. alterniflora stems as a refuge from predation, and to feed on whatever (e.g., detritus, algae, fish larvae, amphipods, tanaids, copepods, and insects) is available (Kneib and Stiven 1978; Weisberg et al. 1981; Werme 1981; Abraham 1985; Moy and Levin 1991; Allen et al. 1994; Kneib and Wagner 1994; Halpin 2000).
Within the ecologically stressed Arthur Kill, the mummichog is considered the only widely abundant fish (Howells and Brundage 1977). It can be an important prey item for protected wading birds and for larger fish such as American eel (Anguilla rostrata), juvenile bluefish (Pomatomus saltatrix), summer flounder, (Paralichthys dentatus), and possibly striped bass (Morone saxatilus) that are common in the Arthur Kill area (Rountree and Able 1992; Parsons 1994; Wilk et al. 1996). Thus, it serves as an important energy transfer mechanism between the Arthur Kill marshes and their adjacent waters (Valiela et al. 1977; Weisberg and Lotrich 1982; Kneib 1986; Kneib and Wagner 1994).
Given the mummichog's close association with salt marshes, it is possible that residual effects of the 1990 oil spill, or any differences among the replanted, unplanted, or reference marshes, may be reflected in its diet. The potential or actual impact of oil on fishery resources and their food have been recognized since the earliest part of this century (Gutsell 1921), but studies on the interactions between these impacts and habitat restoration have just begun. Brzorad and Burger (1994) suggest that the diet of the mummichog in the Arthur Kill has been altered in these polluted waters by restricting the availability of its prey. However, the mummichog is moderately tolerant of oil contamination, except under thermal or osmotic stress (Abraham 1985). The mummichog diet can also suggest pathways by which oil and other anthropogenic contaminants can be transferred from sediments, substrates, and lower biological levels of the marsh ecosystem to higher trophic levels.
Gut content analysis can also support and enhance the results of stable isotope analysis for understanding trophodynamics. For example, Kneib et al. (1980) and Hughes and Sherr (1983) used isotopic analysis to show two main sources of plant carbon being incorporated into mummichog tissue, and Griffin and Valiela (2001) used isotopic analysis to show that the mummichog moves up approximately one step within the benthic trophic food web during a single growing season, consistent with an increase in length. Analysis of stomach contents can define the original and intermediate transfer of these and possibly other sources of plant carbon, as was true for the Griffin and Valiela (2001) study. Examination of the diet of the mummichog, the most common bottom fish along the marsh fringe, contributes to interpreting algal and benthic invertebrate community structures at restoration study sites, as these structures can be altered by the feeding of mummichogs (Vince et al. 1976; Kneib and Stiven 1982).
The functional evaluation of restored salt marshes is important for the assessment and improvement of restoration efforts. Presence or absence of predators such as fish, and variations in their abundances in restored habitats, are valuable indicators of general habitat suitability, but these variables can only suggest that the habitat has recovered its normal ecological function. Trophic relationships and biological energy transfers are considered useful functional endpoints to a restoration, i.e., the diet and feeding of a forage species such as the mummichog can indicate the restoration of an important ecosystem link (Kelly and Harwell 1990). In addition, the mummichog is, as Halpin (1997) states, an "excellent model for studying not only the ecology of saltmarsh fish but rules governing the invasion of marginal habitats by mobile animals."
The mummichog is also an ideal candidate for this study because of its assumed fidelity to the study sites. It is thought to have a limited home range, generally <36 m of shoreline/marsh fringe (Lotrich 1975), although a recent study shows that its home range can be quite a bit larger (Sweeney et al. 1998). However, Halpin (1997) shows that this species is restricted to areas representing a very small proportion of the total available habitat; it also displays fine-scale differences of habitat usage, with patterns of habitat usage varying seasonally (Allen et al. 1994), and appears to remain and feed within the small estuaries in which it was hatched (Griffin and Valiela 2001).
This chapter presents and discusses the results of a preliminary examination of the diets of the mummichogs collected as part of the Arthur Kill study in fall 1996 and spring-summer 1997. The diets of these fish were examined for differences which might be correlated with the replanting efforts following the 1990 oil spill.
METHODS AND MATERIALS
Fish were collected in September 1996 and between May and early August 1997 with standard, tubular, galvanized-wire-mesh killitraps with open funnels at either end, baited with either a fish-based, orange-colored, canned cat food or freshly broken whole northern quahogs (Mercenaria mercenaria) or Atlantic surfclams (Spisula solidissima). The bait was contained within ultrafine synthetic mesh bags. Some trap samples from the Old Place Creek and Con Ed Tower marsh sites were obtained from cooperative collections with SMRT; SMRT used bread as bait in their traps. These baits were readily identifiable in the stomachs, thus allowing separation from the other stomach contents when the fish actually ingested the bait.
Traps were required for collection as seining was not feasible because of conditions at most sites. Since the objective of this study was to examine how mummichogs were using the marshes for feeding, and since the fish mostly fed in the marsh at high tide, the traps were deployed at the marsh fringe just before high tide. Saltmarsh fringes are considered important factors for fish in marsh restorations (Peterson and Turner 1994). The traps were left for 2-3 hr and were retrieved when water had retreated from the marsh surface so that stomach contents of trapped fish would best represent recent feeding on the marsh surface. Butner and Brattstrom (1960) and Allen et al. (1994) reported that mummichog stomachs are mostly full at or after high tide.
Upon trap retrieval, at least 30 mummichogs of mixed sizes were removed and preserved in 10% Formalin. The spring-summer 1997 collections were problematic as an unusually cool spring seemed to retard the mummichog's return to all of the marshes, and required an extended collection period to obtain sufficient samples at all sites. In colder months, mummichogs tend to remain higher up in creeks or in saltmarsh tide pools (Fritz et al. 1975; Smith and Able 1994; Halpin 1997).
In the laboratory, 30 fish were selected from each of the six sites for both collection periods, for a total of 360 fish. Each set of 30 fish contained 10 each of the largest-, smallest-, and intermediate-sized fish. Fish were measured to total length and were sexed, then their stomachs and intestines were removed and examined under a dissecting microscope. The total stomach volume was estimated, then the contents were examined and separated into definable food types. Organismal prey were identified to the lowest practical taxonomic level, their proportional contribution to the total volume was visually estimated, and countable items were enumerated. Because meiofauna were found in the stomachs, the contents were specifically examined for copepods, foraminifera, nematodes, oligochaete fragments, and diatoms. These latter taxa could provide clues to specific microhabitat use, such as feeding on the algal mats growing at the base of S. alterniflora stems (Werme 1981). A variety of estuarine invertebrate taxonomic keys were used to identify stomach contents, e.g., Wilson (1932), Gosner (1971), Bousfield (1973), and Weiss (1995). When bait was detected in the stomachs, it was noted and the upper intestine was also examined in case the bait had pushed natural food out of the stomachs. Gastrointestinal evacuation can be fairly rapid for mummichogs, often on the order of several hours (C.L. MacKenzie, National Marine Fisheries Serv., 74 Magruder Rd., Highlands, NJ, 07732, pers. comm.).
Comparative analysis of the diets among sites and treatments for this preliminary study included the percent frequency of occurrence of specific food items or prey in the stomachs, and the estimated mean percentage of total stomach volume associated with a specific food item or prey. Numerical analysis was not feasible because of the abundance of noncountable material in the stomachs (e.g., detritus, algal material). The small sample sizes, especially per fish length class, and the preliminary nature of the study, precluded the use of extensive statistical treatments.
RESULTS
As the focus of this preliminary study is on possible differences among collection sites and treatments and not on seasonal or interannual variability, the results for each collection period are presented separately.
September 1996
The sizes of the 180 mummichogs examined for this sampling period ranged from 3.3-10.5 cm. This size range was consistent among sampling sites. The results of the stomach content analysis among sites and treatments for this period are presented in Table 16 and Table 17.
The data on percent frequency of occurrence (Table 16) shows that detritus was the most commonly occurring material at all sites and treatments. Algae, both strands (mixed chains of diatoms or variously colored filamentous tubes) and macrophytes, were next in overall, but variable, importance, especially at Con Ed Tower (unplanted), Tufts Point (reference), and Mill Creek (reference) sites. Insects, mostly fragments of adult forms, but including Diptera larvae, were eaten at all sites at a low frequency. Decapod shrimp (mostly Palaemonetes fragments) commonly occurred only at Tufts Point, while the marsh hopper amphipod, Orchestia grillus, commonly occurred only at Mill Creek. The shrimp Palaemonetes sp. was also often collected in the fish traps at some sites, but was found only in the stomachs of the larger fish.
The data on mean percent stomach volume (Table 17) show that detritus was less important and occurrence of bait was more obvious. The use of detritus was highest at the two northernmost sites, Old Place Creek and Con Ed Tower. The occurrence of bait in stomachs was generally highest at all four oiled sites, and is a sampling artifact, although it may indicate limited food availability. Algal strands and blades were important at the two reference sites. Again, decapod shrimp were notable only at Tufts Point, while Orchestia grillus was notable only at Mill Creek. Microscopic items in the stomachs (e.g., foraminifera) appear to be minor contributors to the food requirements of the mummichog.
May-August 1997
The sizes of the 180 fish examined for this sampling period ranged from 3.2 to 9.9 cm, which is similar to the previous sampling period. The results of the stomach examinations for this period are presented in Table 18 and Table 19.
The most frequently occurring item was amphipod fragments, probably Gammarus sp., with detritus being next in frequency; detritus was less frequent at the two reference sites (Table 18). Insects again and nematodes occurred at all sites. Algal strands were found only at the unplanted and reference sites, while algal blades were found only at the oiled (both replanted and unplanted) sites. Some new food items appeared in this collection period, but only the occurrence of some unidentified invertebrate eggs (perhaps crustacean) at Mill Creek was notable. Harpacticoid copepods commonly occurred only at Tufts Point. Items that were relatively frequent in September, but that were rare or undetected in this period, were foraminifera, the marsh snail Melampus bidentatus, Orchestia grillus, decapod shrimp, spiders, and bait.
As contributors to total stomach volume, amphipod fragments and detritus were about equal, combined to make up about half (49.9%) of the total estimated stomach volumes, and were notable at all sites (Table 19). Algal strands were found only at the unplanted and reference sites, while algal blades were found only at the oiled sites and were 2-3 fold higher at the oiled/replanted sites. The reference sites differed from the oiled sites in the volume of detritus, algal blades, insects (i.e., all three items being higher at the oiled sites), and of harpacticoid copepods and invertebrate eggs (i.e., both items basically occurring only at reference sites).
DISCUSSION
There are some suggested differences in the mummichog diets between the replanted (i.e., Old Place Creek and Sawmill North) and unplanted (i.e., Con Ed Tower and Sawmill South) marshes, and between all the oiled marshes and the reference sites (i.e.,Tufts Point and Mill Creek). The following discussion on diets focuses on a few diet items to highlight those suggested differences.
In September 1996 (Table 16), the greatest differences (i.e., greater than or equal to a factor of 2, at levels >5%, with a "+" after a diet item indicating that the highest value for that item was associated with a replanted site) between replanted and unplanted sites were in the mean percent frequency of occurrence of algal strands, algal blades (+), plant fragments, foraminifera (+), nematodes (+), insects (+), Oithonia (+), Gammarus lawrencianus (+), G. mucronatus, and organic matter (+). In 1997, algal strands, nematodes, organic matter, and crab fragments showed strong differences in mean percent frequency of occurrence at the replanted and unplanted sites (Table 18). On a mean percent stomach volume basis, strong differences (i.e., same criteria as those for mean percent frequency of occurrence) between replanted and unplanted sites in September 1996 were noted for algal strands, spiders, and organic matter (Table 17), and in 1997 for algal strands, algal blades (+), organic matter, and Cirolana (+) (Table 19). The other differences among diet items at the replanted and unplanted sites were for items of minor importance (<5%) or were differences less than a factor of 2. The use of the 5% level of importance and a factor of 2 as suggesting meaningful differences is tentative, but probably reasonable given the natural expected variability in diets.
There were greater differences in mean percent frequency of occurrence and mean percent total stomach volume values when comparing the four oiled sites with the two reference sites. For September 1996, there were notable differences (i.e., greater than or equal to a factor of 2, at levels >5%, with a "+" after a diet item indicating that the highest value for that item was associated with a reference site) between the oiled and reference sites in the mean percent frequency of occurrence of algal strands (+), algal blades (+), foraminifera, Melampus bidentatus, insects, harpacticoid copepods, Orchestia grillus (+), and decapod shrimp (+) (Table 16). For 1997, these differences in mean percent frequency of occurrence were notable for detritus, algal strands (+), algal blades, insects, harpacticoid copepods (+), invertebrate eggs (+), and slug-like items (+) (Table 18). In September, notable (i.e., same criteria as those for mean percent frequency of occurrence) mean percent stomach volume differences are suggested for detritus, algal strands (+), algal blades (+), Orchestia grillus (+), decapod shrimp (+), and organic matter (Table 17). In 1997, there were differences for detritus, algal blades, insects, harpacticoid copepods (+), organic matter, invertebrate eggs (+), and slug-like items (+) (Table 19).
Overall, the diets of the mummichogs collected in the Arthur Kill at the various sites appear similar to the diets reported in previous studies (e.g., Vince et al. 1976; Kneib et al. 1980; Abraham 1985; Joyce and Weisberg 1986; Allen et al. 1994). Much of the material or prey found in the Arthur Kill mummichog stomachs, such as the algal strands, insects, spiders, marsh snails (Melampus bidentatus), marsh amphipods (Orchestia grillus), Gammarus, decapod shrimp, and the detritus may have easily been obtained within the flooded marsh, the small drainage channels on the marsh surface, the marsh fringe, and in the adjacent marsh creeks and channels. The seasonal differences in the diets are expected and have been reported in other studies (Werme 1981; Valiela et al. 1977).
The relatively high mean percent frequency of occurrence and/or mean percent stomach volume levels for detritus, algae, and other plant material at all sites may indicate a poor diet (Prinslow et al. 1974; Targett 1979; Kneib et al. 1980; Allen et al. 1994; Brzorad and Burger 1994). The Arthur Kill suffers from multiple-source pollution, and previous studies have demonstrated that mummichogs from Piles Creek, a mercury-polluted tributary of the Arthur Kill, show reduced longevity and rates of prey capture, feeding, growth, and fin regeneration, as well as increased vulnerability to predation by blue crabs, compared to conspecifics from uncontaminated reference sites (Toppin et al. 1987; Weis and Khan 1990, 1991; Smith and Weis 1997). The guts of fish from an unpolluted site on the southern New Jersey shore contained five times as much freshly killed prey by weight as those of fish from Piles Creek, and twice the amount of shrimp (Smith and Weis 1997). The vast bulk (85%) of the Piles Creek fishes' diet consisted of detritus. However, Allen et al. (1994) suggest that the ingestion of detritus and algae may sometimes be deliberate and may contribute to the nutrition of the fish, and Jeffries (1972) concluded from fatty acid analysis of mummichog gut contents and muscles that a reasonable diet for this species included five times as much detritus as marsh invertebrates. Moy and Levin (1991) found that the diet of mummichogs from a created marsh in North Carolina consisted mostly of polychaetes and algae, while a large percentage of the diets of fishes from natural marshes consisted of detritus and insects. Moy and Levin (1991) suggest that this difference in diet is due to differences in macrofaunal composition between their natural and created sites; oligochaetes were actually abundant in the natural marshes, but inaccessible to the mummichogs. This may be true for the mummichogs from the Arthur Kill, as oligochaetes were fairly abundant as macrofauna at all of the sites except perhaps for Saw Mill Creek North (see Chapter VI, "Benthic Invertebrates"), but were almost completely absent from the stomachs. In any case, it appears that the highest use of detritus in this study was in the northern reaches of the Arthur Kill.
The preliminary results and discussion presented here are just that, and a more detailed analysis would be required to confirm that any of the differences among the sites suggested here were significant or real and related to habitat quality or to the replanting efforts.
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