Abstract

The jellied condition in the west coast Dover sole is one in which fillets contain excessive moisture and below normal protein levels. Incidence of excessively jellied fillets is as high as 60 percent in some catches. 100 fish were obtained at sea once a month over a one year period, and examined histologically, ultrastructurally, and physiologically. Parameters studied included depth of capture, seasonality, sex, reproductive stage, age, length, weight and fillet percent moisture content. Myxozoan protozoan infestation was ruled out as a cause, but gonadal maturation and cellular disorders related to muscular dystrophy have not been ruled out as a cause for jelliness. Ecologically, the jellied condition is most prevalent in Dover sole captured on their spawning grounds in deeper water (over 650 meters). No immature fish are jellied regardless of depth of capture. Histologically, the jellied condition is characterized by an increase in intercellular space, a water-logged appearance following staining, an osmotic bloating of muscle nuclei, and an infiltration of fibroblasts leading to deposition of connective tissue. Ultrastructurally, the jellied condition is characterized by an increase in intracellular space and an uneven distribution of myofilaments within muscle cells.

The fishery for Dover sole, Microstomus pacificus (Lockington), originated in the 1940's, concentrating at that time in the nearshore waters off the northern California coast (1). As the demand for this product increased, the Dover sole bottom trawl fleet also increased in size and improved technology, resulting in an eventual depletion of nearshore stocks in recent years. This nearshore depletion of Dover sole, combined with more efficient deep water gear and rough weather boats allowed for a deep water fishery (up to 1000 meters) to target the large spawning aggregations which migrate from nearshore waters to deep water during the winter months (2). As the deep water fishery for Dover sole increased, there is significant increases in the percentage of fish landed which exhibit the "jellied" condition. This represents a serious economic burden to both commercial fishermen and fish processors, as jellied fillets are rejected by most sources (3).

The jellied condition in Dover sole is characterized by a slimy, jelly-like, glossy, and quivering (when touched) flesh. Higher water content and lower than normal protein content are also indicative of the jellied sole fillet (4). These abnormal characteristics result in an interior, if not unacceptable, cooked product. Traditionally, Eureka Fisheries, Inc., alone has annually sold near 350,000 pounds of Dover sole fillets to the military. With the increasing incidence of the jellied condition, they cannot produce a fillet product to Meet current military standards for sole fillets (5).

National Marine Fisheries Service and California Fish and Game Department have conducted preliminary studies on the jellied condition in Dover sole, but environmental factors associated with the condition and its cause were not addressed quantitatively. Preliminary studies have been conducted to characterize the histology And ultrastructure of the jellied condition in Dover sole. In this study, the presence or absence of the jellied condition in Dover sole is analyzed with respect to sex, state of maturity, length, weight, age, time of year, geographic location and depth of capture.

Possible causes of the jellied condition are examined using standard histological, bacteriological, and parasitological techniques. Statistical analysis of all project findings are providing information to commercial fishermen as to how to minimize amounts of jellied sole in their catches, and therefore maximize their profits in the presence of the jellied condition.

Materials and Methods

Twenty-five to fifty whole, live Dover sole were randomly sampled from both a shallow water trawl (less than 180 meters) And a deep water trawl (650-1020 meters). The sampling was done every 4-6 weeks as a part of a regular commercial trawling trip. Dover sole randomly selected were put into plastic buckets and stored on ice in the hull of the boat, typically for 2-3 days. Parameters noted while on board the fishing vessels were: date, depth, and Loran location of capture.

Upon return to the laboratory, individual whole Dover sole were measured as a standard length (SL) in centimeters and a wet weight in grams recorded for each individual,

For age determination, several dozen scales were removed from the eyed-side of the body of each fish, according to Hagerman (1). Otoliths were also removed for comparison and validation of scale ages.

Fillets were then removed from each side of the fish, and a wet weight in grams recorded after the skin was removed and fillet trimmed. One fillet was then placed in an aluminum oven pan and placed in a drying oven at 100-110ºC until their weight stabilized (48 hours). The dried remains were weighed and fillet percent moisture content was determined. The fishes other fillet was used for bacteriological and parasitological analyses.

Sex of each individual was determined by dissection and visual inspection of the gonads. Reproductive stage was classified according to Hagerman (1) by gross examination.

Parasitological analysis of jellied Dover sole fillets were done using Standard techniques. Strong emphasis was placed on parasites of the musculature after initial external and gut examinations showed no abnormal populations of parasites. In addition, thorough analysis of muscle tissue for (Myxozoan) protozoan spores and cysts was performed.

Results

The two most significant correlations found to be associated with the jellied condition are depth of capture (see Table 1, 2) and reproductive state (see Table 3). If 90.0% moisture content in the fillet is used as a cutoff point for normal to moderately jellied fillets, exactly 50% of all Dover sole fillets sampled from deep water exceed this cutoff point and are classified as excessively jellied (see Table 1).

Table 1. Composition of Deep Water Samples of Dover Sole Fillets (Depth of Capture 650 Meters to 1020 Meters)

Seasonal variations relating the depth of capture with the jellied condition occurred in deep water, but did not occur in shallow water. Virtually all shallow water fillets examined with the exception (if one female, contained less than 90% moisture in the fillet (see Table 2).

Table 2. Composition of Shallow Water Samples of Dover Sole Fillets (Depth of Capture Less than 180 Meters)

In comparing the different reproductive stages with the jellied condition, there are several significant findings (see Table 3). Virtually no immature fish are jellied, regardless of capture depth. The average fillet percent moisture value for all immature males and females is 85.1%. The reproductively mature categories contain significantly higher percent moisture values hovering near 90%. Jelliness is a problem in both mature males and mature females, but large, gravid and spawning female Dover sole exhibit the highest level of fillet percent moisture content.

Table 3: Composition of Reproductive States as a Percent of Total Project Sample and Average Fillet Percent Moisture Values at All Depths Combined.

Breakdowns of length/weight categories show no significant correlation with jelliness, although, again, the larger and older fish exhibit the phenomenon more often than smaller mature fish.

Parasitological and bacteriological analyses of jellied Dover sole muscle tissue have virtually eliminated parasitic organisms and bacterial agents as a possible cause of the jellied condition. Myxozoan cysts (Kudoa clupeidae) were found in just slightly more than one percent of all fish sampled.

Discussion

Evidence gathered from the results of preliminary work on characterizing the jellied condition combined with results from this project point to flatfish reproductive strategy as the primary cause of the jellied condition in Dover sole. This supports the findings of Roff (6,7) and MacKinnon (8) that if muscle protein is mobilized to provide for gonadal maturation and maintenance through periods of cessation of feeding associated with spawning, the result would be decreased protein and increased moisture in the muscle tissue.

The fact that no immature fish were found to be jellied is a significant one. Likewise, the finding that virtually no fish sampled from shallow water are jellied (except for one female) is significant. Since Dover sole migrate from shallow water to deep water to spawn in the winter and early spring, if the condition were related to spawning, one would expect to find a significant proportion of fish in deep water to be jellied, and few, if any, jellied Dover sole in shallow water. This is precisely what was found.

If the jellied condition is, indeed, a reproductive strategy for gonadal maturation in Dover sole, it will he necessary for northwestern U.S. bottom trawl fishermen to fish Dover sole only during non-spawning months. Other species should be targeted during the winter deep water spawning months.

However, it is commonly known that Dover sole, by nature, contains higher fillet percent moistures than other flatfish. Processors have hopes to use quantified data on jellied Dover sole in order to persuade government seafood code regulations to allow Dover sole to be considered under seperate inspection requirements.

Observations of dystrophic-like conditions seen in jellied Dover sole muscle tissue suggests the possibility that a type of muscular dystrophy is causing muscle breakdown seen in the jellied condition. Further research with silver staining of neuro­muscular joints is planned to study these observations.

Acknowledgments

This work is a result of research sponsored in part by NOAA, National Sea Grant College Program, Department of Commerce, under grant number NA80AA-D-00120, project number R/F-89, through the California Sea Grant College Program, and in part by the California State Resources Agency. The U.S. Government is authorized to reproduce and distribute for governmental purposes.

The authors wish to thank the crews of the F/V DONNA MAE and F/V ANNA W for their endless assistance and interest, as well as Eureka Fisheries, Inc., for technical assistance, providing fish, securing fishing vessel cooperation, University of California Humboldt Marine Advisor Chris Toole for technical assistance, National Marine Fisheries Service (National Seafood inspection Lab) and Paul Comar for technical assistance, California Department of Fish and Game (Marine Resources Laboratory, Eureka, CA) for assistance in all phases, Tom Lazio Fish Company, Pacific Biological Station (Nanaimo, British Colombia), U.S. Fish and Wildlife Service, and the University of Florida.

Robert A. Fisher assisted with all phases of this project, as well as Joeseph Poulos, Scott Barrow, and Dave Fuller (undergraduate assistants). For their help we are grateful.

References

1.  Hagerman, F.B. The biology of the Dover sole, Microstomus pacificus (Lockington). Calif. Dept. Fish & Game, Fish Bull. No. 85. 48 pp (1952).

2.  Pacific Fishery Management Council. Fishery management plan and supplemental environmental impact statement for the Washington, Oregon, and California groundfish fishery. 20 pp (1982).

3.  Quirollo, L. Personal communication. California Department of Fish and Came, Marine Resource Laboratory, Eureka, CA 95501. (1984).

4.  Patashnik, M. and Groninger, H.S. Observations on the milky condition in some Pacific coast fishes. J. Fish. Res. Bd. Canada. 21: 335-346 (1964).

5.  Comar, P. Field report on efforts to quantify the jellied condition in west coast Dover sole. National Marine Fisheries Service, National Seafood Inspection Laboratory, Pascagoula, Mississippi (1984).

6.  Roff, D.A. Reproductive strategies in flatfish: A first synthesis. Can. J. Fish. Aquat. Sci. 39: 1686-1698 (1982).

7.  Roff, D.A. An allocation model of growth and reproduction in flatfish. Can. J. Fish. Aquat. Sci. 40: 1395-1404 (1983).

8.  MacKinnon, J.C. Summer storage of energy and its use for winter metabolism in gonad maturation in American plaice (Hippoglossoides platessoides). J. Fish. Res. Bd. Canada. 29: 1749-1759 (1972).