RESEARCH METHOD DEVELOPMENTS' FOR ASSESSMENT OF' SUBTIDAL COMMERCIAL' MACROPHYTES

Transcription

1 RESEARCH METHOD DEVELOPMENTS' FOR ASSESSMENT OF' SUBTIDAL COMMERCIAL' MACROPHYTES R.E. Semple Invertebrates and Marine Plants Division Fisheries Research Branch Department of Fisheries and Oceans Scotia-Fundy Region P.'0. Box 550 Halifax, Nova Scotia, Canada B3J 2S7 The marine alga Chondrus crispus is an important natural resource and has been the SUbject of numerous ecological studies. Commercially harvested beds are located on a shallow sandstone~type bottom, approximately 1-10 m deep and up to 1.0 km wide. Qualitative and quantitative techniques have been employed which utilize random sampling grids, point transects (both permanent and temporary)" and photography to assess the biomass, substrate, and other related parameters. Techniques and equipment were improved and adopted to suit individual studies. Rapid methods of laying transects, transporting fully suited divers via surface tow to sampling sites quickly, using a portable underwater suction sample, and relocation of submerged objects are discussed. INTRODUCTION The marine alga Chondrus crispus Stackhouse (Irish Moss), has been an important inshore fishery in the Canadian Maritime Provinces since the second world war (MacFarlane 1968). Carrageenan, the extract from the Irish Moss, is used as a food emulsifier. The importance of this seaweed led to an increase in harvesting (Pringle 1981) and hence the need for resource management. Various studies were carried out, designed to provide the biological advice required for management. Some of the information required was as follows: 1) distribution of commercial beds; 2) biomass estimates; 3) density; and 4) rate of growth and recruitment. Chondrus is a bushy plant, generally 8~15 em tall. Commercial densities of Chondrus around Prince Edward Island (Fig. 1) are found in depths ranging from m and in beds up to 1 km-in width. The plants are contagiously distributed (Pringle, unpublished data) within each bed on various types of sandstone'substrate. The shallowness of the seaweed beds result in ice scouring during the winter months and turbid water from the prevailing winds in the spring and autumn. Water temperature ranges from -2 C to 23 C hence, field work must be carried out quickly and efficiently. Qualitative data on the distribution of commercial biomass was collected by towed vechicles (Judson and Gorazdowska 1967) and underwater photography (Taylor 1972). Percent cover within commercial beds was determined along transect lines by SCUBA equipped divers (Pringle and Semple 1983). Biomass estimates were made by point 228

2 ASSESSMENT OF SUBTIDAL MACROPHYTES - SEMPLE Gulf of 51. Lawrence o 50km New Brunswick Nova Scotia Figure 1. o The southern Gulf of St. Lawrence (46 S2'47.2"N, 6S 14'2.6''W) showing the Maritime Provinces of Prince Edward Island, New Brunswick and Nova Scotia. transect surveys (Johnson and Herring 1968; Judson and Gorazdowska 1968) and stratified random surveys (Pringle, unpublished data); both employed quadrats. Information on both density and size structures were collected within quadrats. Growth was obtained from outplants placed on submerged concrete platforms (Hanic and Pringle 1978). These studies were both time consuming and thus, expensive. Diving under adverse conditions of low water temperatures and poor visibility, along with working amongst dragrakers (for a description of harvesting techniques see Scarratt 1972 and Pringle et al 1981) creates the need to minimize the time spent underwater and to increase the speed and effectiveness of the survey. Some of the methods and aids developed to assist in shallow submarine benthic ecology is presented and discussed. TECHNIQUES Point Transect Survey Judson and Gorazdowska (1967) developed the basic methodology employed here to determine biomass and distribution around Prince Edward Island. Transects along the coast~line were demarcated by directional shore markers (see Fig. 2). Samples were collected from stations at regular intervals along the transect line. 229

3 ASSESSMENT OF SUBTIDAL MACROPHYTES - SEMPLE ) '..;.' '~~ ;: SHORE.:. MARKERS Figure 2. Overhead schematic of the buoy-laying method used in the point transect technique. One of the problems in this survey involved the positioning of the buoys along these transect lines. A "buoy placement reel," was operated by anchoring a polypropylene line at the inner buoy station and releasing buoys at designated marks on the line. This proved unsatisfactory for the following reasons: 1) the current deflected the line; 2) harvesting dragrakes tangled the line; and 3) considerable time was spent rewinding the line on each transect. A simpler, more effective method was devised. A boat towing a buoy on a line (equivalent to the length of the station intervals) travelled along the transect line (Fig. 2). When the towed buoyl came abreast the previous station's buoy2, crew would discharge another buoy3 to demarcate the next station. This operation continued until the outer limits of the seaweed bed was exceeded. method insured that the buoys were positioned at the proper intervals and deployed rapidly. This leap~frog Another logistic problem associated with the point transect technique can be the constant diver entry and reentry to the water at the various stations. This can be tiring and inefficient if the experimental design requires numerous stations. Consequently, a tow board (1.2 x 2.4 m) was designed to transport fully equipped divers on the surface between stations (Fig. 3). The diver would lay on the board to be towed between stations. Boat crew would drop the sampling equipment at each station. This resulted in a savings of both diver energy and survey time. The original tow board was constructed with a hollow interior and waterproofed with fibreglass. We found that this was too heavy for easy deployment and a single sheet of 2 cm thick plywood with reinforced ribs was sufficient. It submerged with a diver in position but planed easily under power (Fig. 3). 230

4 ASSESSMENT OF SUBTIDAL MACROPHYTES - SEMPLE Figure 3. Random Stratified Technique The diver tow board in operation. There was a need for a more accurate method of obtaining biomass estimates. A random stratified survey was developed to replace the point transect survey (Pringle, unpublished data). Sampling areas, 4,900 m 2 were randomly chosen throughout the seaweed bed. Each square was divided off in a grid pattern of fourty~nine 100 m 2 (Fig. 4) and 20 squares were randomly chosen for sampling. The first method developed to demarcate the 4,900 m 2 's involved a lead-core nylon line with a surface buoy on each corner. A diver would position the square on the bottom and then place buoys with anchors, 10 meters apart around the perimeter. The buoys were to form a reference point for each sample in the grid. -For example, the square in which the diver is sampling (Fig. 4) is buoy 4, adjacent buoy 3. This method of setting up the square took considerable amount of diving time and the wind and/or currents often moved the buoys out of position. This method was discontinued and the three sided roped area method on the surface (Fig. 4) was implemented. The open ended square consisted of a buoy with concrete building blocks at each corner with six smaller buoys per side. Diving was no longer needed to set out the square and it could be deployed in about five minutes from the boat. By positioning the open end downwind, the boat could keep from drifting over the lines and the boat crew could easily enter the square to position the sampling quadrats and assist the divers. Collection Methods The seaweed samples collected by divers were usually handpicked if population structures was determined, or handraked if harvestably mature fronds only, were required. Sample units were scraped from the substrate if all prerecruited (including juveniles) fronds were required. The immature fronds (1.0 mm to 2.0 em) were difficult to put in and retain in typical mesh collecting bags. One technique used was to insert a fine mesh liner into the bags. Another technique involved a benthic suction sampler (Foreman pers. comm.*). 231

5 ASSESSMENT OF SUBTIDAL MACROPHYTES - 2 Figure 4. SEMPLE 3 Overhead schematic diagram of the randomly stratified technique showing a diver sampling an m2 within a randomly chosen 100 m2 Itconsisted of A8S plastic pipe (100 x 10 cm) which had a lever. controlled air supply connected to the second stage of the diver's regulator. The sample units were air~lifted to fine mesh collecting bags (nylon stockings) as they were scraped off the various substrates. Few fronds were lost using this effective method. Growth Studies A method of out planting Chondrus was developed to follow growth and recruitment (Hanic and Pringle 1978). Chondrus was grown on pottery units and placed in situ on concrete platforms bolted to the ocean floor. Originally the units were attached individually but trays holding as many as eight units were designed to slide into grooved indentations on the block. This greatly assisted in the collection and replacing the outplants. Relocation Methods The outplanting substrates wer"e located on commercially harvested seaweed beds. They could not be continually buoyed due to moving ice, wind generated wave motion, and dragrakes. The platforms, therefore, had to be frequently relocat"ed by di vera..sear-ch condi tiona 232

6 ASSESSMENT OF SUBTIDAL MACR.OPHYTES - SEMPLE were poor due to the lack of bottom features and the poor visibllityo Triangulation, by using unique permanent shore objects, was difficult In some areas because the shore~line was wooded and featureless. Shore markers were set up using angles taken from permanent bench marks. As the units were a considerable distance from shore, triangulation from this method, still was not accurate. Electronic pinger receiver system was used to help to relocate units. The acoustic transmitter (SR69 Tag by Smith-Root Inc*) was a type similar to those used in salmon tagging. The sonic receiver (Smith-Root Model TA-60) was placed in a waterproof housing. The transmitter had replaceable batteries which had an operating life of six to eight months (had to be longer than the ice cover). The individual transmitters operated at different frequencies, so several could be used in the same vicinity. The transmitters in the concrete platforms, permitted rapid relocation by divers. This was especially helpful in the early spring after the ice break4 up when searching in cold water becomes an important time factor. DISCUSSION Time is an important factor in any type of field work where SCUBA is involved. Bad weather in the study area delays not only the immediate surface operations but can also delay diving for several days due to poor underwater visibility. The southern Gulf of St. Lawrence is wind swept and western Prince Edward Island bears the brunt of the prevailing winds thus, it is important to be efficient during good weather windows. Sometimes it takes a problem or lack of equipment to force one to evaluate methodology. The method of laying buoys was developed in Newfoundland because a reel was unavailable for the survey work there. The slide trays method for the outplants was developed by a diver whose dry suit leaked in the cold water, while collecting the individual units. New equipment and methods are continually improving working conditions and making field work more efficient. One should be frequently assessing field methods and be watchful for helpful short-cuts. ACKNOWLEDGEMENTS I would like to give special thanks to D. Roddick whose underwater skill and inventive ideas added much to the effectiveness of the various projects over the past years. And to J.D. Pringle who instigated much of the scientific projects mentioned here. *Dr. R. Foreman, Director, Banfield Marine Station, Banfield, British Columbia. *Smith Root Inc., Northeast Salmon Creek Ave. Vancouver, Wash. U.S. A

7 ASSESSMENT OF SUBTIDAL MACROPHYTES - SEMPLE LITERATURE CITED Hanic, L.A. and J.D. Pringle Outplant method for phenological studies of Chondrus crispus in mechanically harvested beds. J. Fish. Res. Board Can. 35: 336~338. Johnson, H.D. and F.L. Herring A Quantity survey of Irish mobs and other seaweeds in the Mlminegash area. Proj. Rep. No. 16, Ind. Dev. Service, Dept. Fisheries Canada; Ottawa. Judson, D.B. and A. Gorazdowska Irish moss and other seaweeds survey ores. Dev. Br., Provo Dept. Fisheries Tech. Rep. 51, Charlottetown, P.E.I. Judson, D.B. and A. Gorazdowska Irish moss and other seaweeds survey "1968. "Res. Dev. Br., Provo Dept. Fisheries Tech. Rep. 56, Charlottetown, P.E.I. MacFarlane, C Chondrus crispus Stackhouse Res. Found., Halifax, N.S.: 47 p. ~ a synopsis. N.S. Pringle, J.D The relationship between annual landings of Chondrus dragrakes, effort, and standing crop in the southern Gulf of St. Lawrence. In: Proc. of the Tenth International Seaweed Symposium. Walter de Gruyter and Co. Berlin. Germany Pringle, J.D Efficiency estimates for various Quadrat sizes used in benthic sampling. Can. J. Fish. AQuat. Sci. Vol. 41 No. 10: 1485~1489. Pringle, J.D., D.J. Jones and P. Rowe Fishing power and ecological impact on Gulf Chondrus (Irish moss) of modified Chondrus dragrakes. Can. MS Rep. Fish. AQuat. Sci Pringle, J.D. and R.E. Semple A description of the major commerctal Irish moss (Chondrus crispus Stackh.) beds off western Prince Edward Island. Can. MS Rep. Fish. AQuat. Sci Scarratt, D.J Investigations into the effects on lobsters of raking Irish moss to Fish. Res. Board Can. Tech. Rep. 329~20 p. Taylor, A.R.A A basis for the continuing assessment of natural and exploited populations of Chondrus crispus Stackh. In: Proceedings of the Seventh International Symposium, Tokyo: Uni. of Tokyo Press, N. Nisizawa (Ed.)