ICES CM 2001/P:24 Assessment of density and biomass of ocean quahog, Arctica islandica, using a hydraulic dredge and underwater photography Gudrun G. Thorarinsdóttir Stefan Áki Ragnarsson The stock of Arctica islandica in Iceland has been assessed using a commercial hydraulic dredge and underwater photography. Abundance estimates based on count of siphons from underwater photographs were much higher than from analysis of the dredge catches. Furthermore the efficiency of the two hydraulic dredges was investigated in tow locations giving different results. The movement of the clams down into the sediment can affect the dredge efficiency and also cause underestimation of stock size based on underwater photographic survey. Keywords: Arctica islandica, stock assessments, dredge efficiency, sea-bottom photograpy Introduction The ocean quahog is distributed all around Iceland and the stock has been assessed at 5-50 m depth in west, north and east Iceland (Eiríksson 1988, Thorarinsdóttir and Einarsson 1996) but the fraction of the resource inhabiting deeper water is unknown. Study of a population off north-west Iceland demonstrated a lifespan of up to 201 year (Steingrímsson and Thorarinsdóttir 1995). Fishery of ocean quahogs for human consumption in Iceland began in 1995 and annual landings have since been 1500 to 8000 tones. A harvesting strategy of 2.5% of the estimated stock is used. The assessments of ocean quahog resources in Icelandic waters have been carried out using a hydraulic dredge. In such study it is very important to know as accurately as possible the efficiency of the dredge used. However, very few adequate direct measurements are available (Medcof and Caddy 1971, Anon., 1998). 1 Many studies have been carried out using underwater photography techniques to estimate abundance of epifaunal bivalves, such as scallops (Langton and Robinson, 1990, Stokesbury and Himmelman, 1993, Goshima and Fujiwara, 1994) but to our knowledge, only one study has estimated abundances of infaunal bivalves (Wigley et al., 1970). Identification of infaunal bivalves from photographs depends solely on siphon characteristics and the general appearance of the protruding parts of the shell, but these characteristics can be very distinctive (Siferd and Welch 1992). In the present study the efficiency of a hydraulic dredge was determined directly with diver observations. Furthermore, an assessment of ocean quahog density and biomass was carried out simultaneously using sea-bottom photography and hydraulic dredge. The aim was to compare these two methods used for estimating abundance of ocean quahog and to estimate the efficiency of the two hydraulic dredges used. Material and methods A study of the efficiency of a hydraulic dredge was carried out in June on sandy bottom at 10 m depth in northeast Iceland. The dredge had an overall dimension of 735x150x365 cm (l x h x w) and had a 305 cm wide and 8 cm deep cutting blade beneath The bar spacing in the dredge was 33 mm, and the water pressure of the pump was 7.5 bars. The towing speed was 2.4 miles per hour and the duration of the tow was 5 minutes. In order to determine the efficiency of the dredge, the distance covered by the dredge was calculated. Ocean quahogs from the catch were weighed, and the total catch weight was divided by the size of the area covered in the tow to give biomass caught in kg m-2. Individuals in a subsample of 25 kg were counted measured and weighted. Ten hours later a diver photographed the dredge track using an underwater videocamera and subsequently collected all clams within five frames of one square meter each randomly located within the track. These clams were counted, measured and weighed. In order to assess the density of ocean quahog by underwater photography a study was carried out at 14 m depth in northwest Iceland in July using a 35 mm vertically oriented underwater camera (Photosea 1000A) placed on a steel frame. During photography, the frame was repeatedly lowered to the bottom at interval of 15 – 30 seconds depending on speed of drifting and a photo was taken. The camera and 2 the strobe were triggered at constant distance above the bottom and therefore the area of the seabed covered in each photograph was always the same. Estimates of the density of ocean quahog was based on counts of siphons which is the only parts of the clam which could be seen on the photos. Subsequently dredge hauls were made in the same location using a hydraulic dredge. The dredge had an overall dimension of 590x103x230 cm (l x h x w) having a 105 cm wide and 9 cm deep cutting blade. The bar spacing in the dredge was 34 mm, and the water pressure of the pump was 8.5 bars. The towing speed was 2.0 miles per hour and the duration of the tow was 10 minutes. The total catch weight was divided by the size of the area covered in each tow to give biomass caught in kg m-2. Abundance indices (mean number/tow) are calculated as the mean weight of a tow divided by the mean live weight of individuals in catches from the same area. The efficiency of this dredge was estimated by comparing the density observed from the photographs and the density calculated from the catches. Results In the study carried out in northeastern Iceland, on the efficiency of the hydraulic dredge, the estimated mean biomass of ocean quahogs in the dredge catch was 1.8 kg m-2, equivalent to 12 individuals (mean live weight of an individual caught by the dredge was 145 g). Diver samples revealed that the mean biomass left in the track was 1.0 kg m-2 and 50 individuals, giving the mean efficiency of the hydraulic dredge, 64% by weight and 19% by number. Due to selectivity of the hydraulic dredge, 85% of the clams caught ranged in shell length 70-95 mm, the mean shell length was 82 mm and none clams were smaller than 30 mm (Fig. 1). Fifty percent of the individuals caught by the dredge had broken shells. The mean length of the clams remaining in the dredge track was 38 mm, the shell length ranging between 10-80 mm, but 93% had shell length <65 mm (Fig. 2). Of all ocean quahogs remaining in the track, 14% had broken shells. Out of total of 180 sea-bed photographs analysed, from northwestern Iceland, ocean quahog was present in 97% of all photos (Fig. 3) with average abundance of 52 individuals m-2, equivalent to 6.1 kg (mean live weight of an individaul caught by the dredge was 118 g). Two types of sediments could easily be distinguished, fine sand (81% of photos) and medium sand. Numbers of ocean quahogs were significantly 3 greater in fine sand (61 individuals m-2) compared to medium sand (43 individuals m-2) (one-way ANOVA (df=380, F=34.7, p<0.0005)). Ocean quahog was the only bivalve species observed in dredge samples and the only species seen on the photographs. The mean biomass from dredge samples was 1.6 kg m-2 and the calculated mean density was 14 individuals m-2 (SD=32). Assuming the average density of ocean quahog 52 individuals m-2 , the efficiency of the dredge was calculated 27% in terms of numbers. The length-frequency distribution of ocean quahog caught by the dredge is shown in figure 4. The mean shell length was 76 mm and about 98% of the clams ranged in shell length 60-90 mm and none were smaller than 45 mm. Discussion In the present study the mean efficiency of the hydraulic dredge used in northeastern Iceland was estimated to be 64% by weight but only 19% by number. The dredge was very selective, catching mainly quahogs in the size range 70-95 mm. Majority of the clams remaining in the track were unbroken and had shell size < 55 mm. The density of ocean quahog was estimated by counting siphons from underwater photographs in northwestern Iceland. Analysis of survey dredge catches in the study area has shown that no other large bivalve species were found and for that reason, siphons of ocean quahogs were easily identified. The diameter of the smaller bivalve species that have been recorded in the fjord (Gardarsson et al. 1980), is too small to be seen on the photographs. In the present study, abundance estimates of A. islandica based on siphon counts from the photographs were more than 3 times higher than estimates from dredge survey carried out at the same time, resulting in dredge efficiency of only 27% in terms of number. The catching efficiency is expected to be higher in terms of weight as the individuals that escape catching are on the average smaller than those taken by the dredge. Very few studies have been carried out estimating the efficiency of hydraulic dredges catching ocean quahogs. Medcof and Caddy (1971) estimated the efficiency close to 100% whereas results from another study gave the mean efficiency by weight 43% (Anon 1998). Dredge efficiency is likely to vary with factors such as sediment type, current speed, the ratio wrap 4 length:water depth, towing speed, hydraulic pressure of the jet, behavior of the clams and possibly the size of the dredge. Estimates of ocean quahog based on counts from photographs and analysis of dredge cathes can be inaccurate. Ocean quahog is known to be able to burrow deep into sediments (self-induced anaerobiosis), until siphons are no longer visible on the sediment surface. Taylor (1976) showed that 36% of individuals remained below the sediment surface over a 25 days study period but found little synchrony among animals in either initiation of the burrowing or the return to the surface.