Survival and growth of cut vs hooked commercial in the Florida Keys

Item Type monograph

Authors Stevenly, John; Sweat, Don

Publisher Florida Sea Grant College

Download date 27/09/2021 22:47:01

Link to Item http://hdl.handle.net/1834/18012 Technical Paper No. 38

Survival and Growth of Cut vs Hooked Commercial Sponges in the Florida Keys

by John Stevely and Don Sweat

I"U)RIM =A GRANT COLL€G€ StEVIXU AM) GRCWTH OF CUT vs . HOOKED COMMERCIAL SPONGES IN THE FU3RIDA KEYS

John Stevely and Don Sweat

Florida Sea Grant Extension Program 117 Newins-Ziegler Hall University of Florida Gainemille, FL 32611

Project No. IR-82-15 Grant No. NA8OAA-D-00038

Technical Papers are duplicated in limited quantities for specialized audiences requiring rapid access to infonoation. They are published with limited editing and without formal review by the Florida Sea Grant College Program. Content is the sole responsibility of the author. This paper was developed by the Florida Sea Grant College Program with support frcm NQ7Ul Office of Sea Grant, U.S. Department of Ccmnerce, grant nwlber NA8OAA-D-00038. It was published by the Sea Grant Extension Program which functions as a aanponent of the Florida Cooperative Extension Service, John T. Woeste, Dean, in conducting Cmpxative Extension work in Agriculture, Hme Economics, and marine Sciences, State of Florida, U.S. Department of -ce, and Eoards of Cormty Ccmaissioners, cooperating. Printed and distributed in furtherance of the Acts of Congress of May 8 and June 14, 1914. The Florida Sea Grant College is an Equal Rployment-Affinoative Action employer authorized to provide research, educational infonoation and other services only to individuals and institutions that function without regard to race, color, sex, or national origin.

TECHNIC& PAPER NO. 38 September 1985 INTRODUCTION

Until the 19401s, the fishery was one of the most valuable fisheries in Florida. However, a combination of the sponge blights of 1939 and 1946 and the introduction of synthetic sponges resulted in reduction of the fishery to a small fraction of its former importance (Stevely, --et a1 ., 1978). In recent years, increasing scarcity of sponges in the sponge beds off of Tarpon Springs has threatened the continued existence of the remaining

Tarpon Springs sponge fleet.

Although an exploratory sponge fishing survey of state territorial waters off of Monroe County (Florida Keys), Florida, had indicated that commercial quantities of sponges were present (Sweat and Stevely , unpubl ished man- uscript), Florida Statute (F.S. 370.17) prohibits use of "deep sea diving apparatus to harvest sponges in this area." This Statute states, "No person may use diving suits, helmets, or other apparatus used by deep sea divers in taking commercial sponges from any waters within the territorial limits of this state."

The purpose of the law was .not to prohibit reasonable methods of harvesting sponges but to protect young sponges. It was enacted to prevent damage to young sponges caused by heavily weighted divers stepping on them.

Early sponge diving gear was of the deep sea variety. Scuba and hookah, common gear today, did not become a viable method of extended until many years after enactment of the law. Light hookah gear with rubber-soled, canvas athletic shoes has now replaced the heavy helmet and lead boot-clad diver.

The Florida Saltwater Fisheries Study and Advisory Committee recommended in 1982 that sponge diving be allowed in the state territorial waters off Monroe County. However, objections were raised concerning the possibility of

overfishing due to increased fishing following legalization of sponge

diving. Therefore, the present project was undertakeil for the purpose of establishing whether a change in harvesting technique -- cutting rather than tearing the sponge loose -- could insure sponge regeneration and thus reduce the possibility of overfishing. Sponges have remarkable regenerative ability;

and if even a small quantity of sponge material is left attached to the

substrate, the sponge may survive and grow back to a commercially valuable

size.

METHODS AND MATERIALS

During June, 1982, a commercial sponge fisherman assisted the project

by locating a bed of sheepswool sponges (Hi ppiospongi a lachne) off of Vaca Key

(Marathon) in the Florida keys (Fig. 1). The site was marked with a buoy, and

Loran C coordinates were recorded to insure relocation of the study site. The

bottom of the site was marked off by laying out a grid of polyethylene line held in.place. by rebar stakes driven into the substrate. Sponges were located

by swimming along the grid lines. Upon locating a sponge, its position was marked by attaching a piece of brightly colored surveyor's flagging tape to the line. The position was also recorded on a plastic chart of the grid pattern. Large calipers (S & S standard radiological ) were used to take two measurements of the sponge's diameter and one measurement of the sponge's hei ght.

After being measured, alternate sponges were either cut loose using a large, sharp knife or torn loose with a sponge hook on the end of a three- foot handle. Sponges were cut as close to the substrate as possible while

attempting to leave a complete sponge base. In practice, this left 1/2 to 1 inch of sponge tissue. The sponge hook was of the type used by commercial sponge divers. When the sponge was removed, the type of harvesting method used was recorded and measurements taken of the remaining sponge base (if there was any left). The exact location of each sponge was marked by driving a rebar stake into the substrate. A numbered, plastic bird band was wrapped around these stakes for identification. Also, in those cases where sponge base was left behind, the sponge base was marked by "sewing" a piece of monofilament line through it and then attaching a plastic bird band. Ten sponges were staked and tagged to serve as a control group. Growth and survival data for experimental and control sponges were collected 12 months

(May '83) and 28 months (Nov. '84) after the study was begun.

RESULTS

Survival of Cut and Hooked Sponges

A total of 69 sponges were either cut (N=35) or hooked (N=34).

Approximately one-thi rd of a1 1 the study sponges (control, cut, hooked) were not found by the end of the experiment for a variety of reasons which are discussed (Fig. 2). The percentages of found sponges for each group of sponges were not significantly different (G-test; Sokal and Rohl f, 1969).

Twelve months after the study began, 70% of the found cut sponges were alive and 39% of the found hooked sponges were alive (Fig.3). The percentage of surviving cut sponges compared to that of surviving hooked sponges was significantly different (G-test; P<.05).

Twenty-eight months after the study began, 71% of the found cut sponges were alive and 41% of the found hooked sponges were alive (Fig. 3). Again, the proportion of surviving sponges between the treatments was significantly different (G-test; Pc.05).

The slightly higher percentage of surviving cut and hooked sponges found at 28 months compared to 12 months was not significantly different and can be attributed to sampling error. Since fewer sponge sites were found at

28 months (Fig. 2.), the sample size used to calculate percentages of surviving sponges was smaller (number of found cut and hooked sponge sites was 55 at 12 months and 47 at 28 months).

On both sampling dates, 100% of the found control sponges were a1 ive.

The percentage of surviving found control sponges was significantly different from the percentage of surviving found cut and hooked sponges (G-test; P<.05).

Growth of Experimental and Control Sponges

Average increase of the maximum diameter of the cut sponge bas was 2.3 cm. (range: -1.5 to 5.0 cm.) after 28 months. Average increase of the maximum diameter of the hooked sponge base was 2.1 cm. (range 0.0 to 4.00).

Only a small number of surviving cut sponges (7, or 39%) and hooked sponges (3, or 33%) were judged to have regrown to a commercial size. Whether sponges had or had not grown to a commercially harvestable size was a qual- itative judgement. Although most of the surviving sponge bases had grown to the point of exceeding 12.7 cm. (to be legal size a sponge must have, when wet, a maximum diameter of at least 12.7 cm), they had not yet grown in height sufficiently enough to be judged as a truly commercially valuable product by the authors. Therefore, only sponges which could be crudely estimated to have attained a height of at least 7.6 cm. were considered as commercial-sized sponges. However, there is no legal requirement for a minimum sponge height.

. The average natural growth rate of control sponges monitored during the study (7 of the 10 original controls were found at the end of the study) was

1.7 cm./year. DISCUSSION

Survival of Cut and Hooked Sponges

This study shows that both cut and hooked sponges can successfully regrow to a commercially harvestable size. However, cut sponges have a substantially higher chance for survival and consequently have a higher chance for regrowth to a commercial size than do hooked sponges. The higher survival rate of cut sponges can be attributed to the fact that cutting insures that some sponge tissue is left attached to the substrate. Sponge tissue is sometimes left behind when hooking the sponge; but often when the sponge is torn free, very little tissue is left. If sufficient sponge tissue is not left, sediment can easily smother the remaining sponge tissue.

It is probable that higher survival of cut sponges than that recorded in this study could be realized if the sponges were cut so that a taller sponge base was left behind, reducing the chances of the sponge being smothered by . . sediment. However, this is economical ly impractical for a commerci a1 sponger at the present time since leaving an additional 3 to 5 cm. of sponge tissue would substantially reduce the value of the harvested sponge. On the other hand, the base of most sponges is embedded with rock and foreign material and normally is trimmed and discarded before retail sale, so an educational, voluntary program directed at sponge producers and buyers might be helpful.

Any management technique based on the requirement to leave a minimum height would be unenforceable since it is impossible to determine at what level a harvested sponge was cut.

If it is assumed that all "lost" sponges represent mortality due to experimental treatment (cutting or hooking) then the survival of a1 1 experimental sponges would be lower than that observed for only sponges (or Figure 2. Percentage of sponge sites found, (number of sponge sites found1 total number of sponge sites) x 100, May 1983 and Nov. 1984.

MAY NOV. 1983 1984

CONTROL CUT HOOKED N-10 N-35 Figure 3. Percentage of sponges found which survived, (number living sponges / number sponge sites found) x 100, May 1983 and Nov. 1984.

MAY NOV. 1983 1984

CONTROL CUT HOOKED N-10 N-35 N-34 the stakes marking their positions) which were found. Complete mortality +, of "lost" sponges was not deemed likely by the authors for the following

reasons: 1) a similar portion (30%) of the control sponges were "lost" compared to the experimental sponges; 2) the sponges were widely scattered over a large area; and with the limitations of underwater visibility, it was impossible to locate all sponges; 3) at times the area was heavily fished by lobster trap fishermen and a number of marker stakes could have been inadvertently dislodged by traps.

Therefore, a maximum estimate of survival would be 71% for cut sponges and 41% for hooked sponges based on data on found experimental sponge sites

(N=47). A minimum estimate of survival would be 51% for cut sponges and 26% for hooked sponges based on number of sponges in the initial experimental group (N=69).

Growth of Experimental and Control Sponges

The results of this study demonstrate that at least 2 years are required for the surviving sponge base to regrow to a minimum commercial size. The observed growth rate would indicate that 1 or 2 additional years would be desirable to insure that a higher percentage of sponges attained harvestable size. The additional time would also allow the sponges to grow to a larger and thus more valuable size, Histori cal ly , several studies of sheepswool sponge growth (Moore, 1910; Crawshay, 1939; Storr, 1964) have measured sponge growth in terms of

vplumetric increases (the number of times a sponge increases in volume during a 1-year period). Variability in sponge shape and repeated attempts to obtain reproducible measurements of sponge height were such that volumetric estimates of sponge size were judged to be inappropriate. Therefore, in this study, sponge growth was expressed in terms of increase in the maximum sponge diameter. Based on repeated measurements of several sponges conducted at the study's beginning, the maximum sponge diameter was the only measurement found to be reproducible within 0.5 cm. These observations have been confirmed in a recent study of sponge growth in Biscayne Bay, Florida (pers. comm., J. Tilmant; Everglades National Park, Homestead, FL) . A1 though di rect comparison is difficult , growth of control sponges in this study appears to be somewhat slower than that recorded in older studies. Storr (1964) measured a growth factor of 2.27 (number of times a sponge increases in volume during a 1-year period) for sheepswool sponges from Piney Point, Florida. Based on this growth rate, Storr estimated it would require at least 3 years to reach a harvestable size. Moore (1910) studied sheepswool sponge growth at Anclote. Key and Sugarloaf Key, Florida, and measured growth factors lower than Storr's data (1.91 and 1.86 respec- tively). Smith (1973) studied a different species of commercial sponge, the grass sponge (Spongia graminea), in card Sound, Florida, and reported a growth factor of '0.77. Studies of sheepswool sponges in the Bahamas and British

Honduras (Crawshay, 1939) found growth rates of 2.34 to 2.94.

Tilmant (pers. comm.) working on sheepswool sponge growth rate in

Biscayne Bay, Florida, reported a 2.0 cm./year increase in diameter. This growth rate would produce a commercial sponge in approximately 6.4 years. The growth rate measured in this study of 1.7 cm./year would produce a commercial sponge in 7.5 years.

The vari abi 1 ity of sheepswool sponge growth rates from different locations is noted in the literature (Stevely, et a1 ., 1978). Substantial differences in growth rates of commerci a1 grass sponges (Spongi a grami nea) 1ocated on the same rock outcropping have been reported (Smith, 1973). It is possible that this study was located in an area not conducive for rapid sponge growth. However, sponges in the area were abundant and appeared to be in healthy condition. Sponge fishermen were seen working the general area, although most sponging activity was conducted closer to Vaca Key in shallower water. Therefore, it seems reasonable to assume that the study site was representative of commercially productive sponge bottom.

CONCLUSIONS

1. Sponge tissue left attached to the substrate after the sponge has been harvested by either cutting or hooking can survive and grow.

2. Survival of remaining sponge tissue is substantially greater when sponges are harvested by cutting compared to those sponges harvested by hooking, since cutting insured that more regenerative sponge tissue was left attached to the substrate.

3. The maximum estimate of sponge survival for cut sponges is 71% and for hooked sponges is 41%. The minimum estimate of sponge survival for cut sponges is 51% and for hooked sponges is 26%.

4. At least 2 years is required for surviving sponge tissue to regrow to a commercially harvestable size. An additional 1 bl- 2 years is required for more sponges to attain a more valuable size.

5. Any attempted management of the sponge fishery requiring, by statute, a minimum height requirement to be left when cutting a sponge would be unenforceable, and that this "recommendation" could be better addressed by an educational program directed at both sponge producers -and buyers. LITERATURE CITED

. CRAWSHAY, L. R., 1939. Studies in the market sponges. 1. Growth from the planted cutting. Jour. Mar. Biol. Assn. of the U.K. 23:553-574.

MOORE, H. F., 1910. The commercial sponges and the sponge fisheries. Bull. U.S. Bur. Fish., Vol . 28, Part 1, 1908:399-511. SOKAL, R. R., and F. J. ROHLF, 1969. Biometry: The principles and practice of statistics in biological research. W. H. Freeman and Company, San Francisco. 776 pp.

SMITH, R. L., 1973. Abundance and diversity of sponges and growth rates of Spongia graminea in Card Sound, Florida. Master's Thesis, Univ. Miami, Coral Gables, Florida. 66 pp. STEVELY, J. M., J. C. THOMPSON and R. E. WARNER, 1978. The biology and utilization of Florida's commerical sponges. Florida Sea Grant College Program Tech. Rept. No. 8. 45 pp. STORR, J. F. 1964. Ecology of the Gulf of Mexico commercial sponges and its relation to the fishery. U.S. Dept. Interior, Fish. Wild. Serv., Spec. Sci. Rept. No. 4666: 1-73.