Dear MARMAM members,
This PhD Thesis was recently submitted and accepted by the University of Haifa, Israel. Title: The Population of Bottlenose Dolphins (Tursiops truncatus), Bottom Trawl Catch Trends and the Interaction between the Two along the Mediterranean Continental Shelf of Israel. For any pdf copies please do not hesitate to contact me at schei...@013.net.il Aviad Abstract General Introduction The present research assesses the relationship between two top predators in the marine food web, the common bottlenose dolphin (Tursiops truncatus) (hereafter CBD), and the bottom-trawl fleet, the main marine fishing industry in Israel, in the framework of the complex relationship between mankind and the sea. Free-ranging, coastal communities of CBD have provided prime research opportunities in the field of cetacean social ecology at several marine localities around the world. Several longitudinal studies of this type have been conducted in the central Mediterranean Basin, notably in the Adriatic and the Ionian seas but none in its easternmost reach, the Levantine Basin. The biological resources in this basin are limited since the level of primary production is low and accordingly the supported food web. The setting where these two top predators are exploiting the same benthic niche in an ultra-oligotrophic body of water sets the scene for competition for limited resources. Competition may first be assessed indirectly by conducting social behavioral research on the CBD population, with emphasis on the relationship with the bottom trawlers. This kind of data may be complemented by following the fish catch trends, particularly whether there is evidence of over-fishing. Then, the question of alleged fisheries/CBD competition could be addressed more directly by comparing features of the CBD diet and the bottom trawl catch. A straightforward comparison could be made between the composition of stomach contents of stranded carcasses and by-caught animals and that of the catch, or inferences may be made by performing stable isotope analysis on CBD tissue and on the tissues of its potential prey. General attributes of the diet of dolphins are reflected through the stable isotope composition of their body carbon and nitrogen: δ13C and δ15N values, respectively. The nitrogen stable isotopic composition (d15N) of tissues provides a powerful tool for determination of the trophic relationships among organisms and the trophic position within the food-web. The carbon isotope (d13C) is useful to the study of diet through its use as a tracer of sources of primary productivity and thus the feeding niche. Primary producers vary in their isotopic C signatures according to their origin (e.g., terrestrial versus aquatic, benthic versus neritic). Aims The first aim of this research was to reveal the different ecological aspects of the local CBD population. Then, following the primary assumption of competition between bottom trawl fishery and the local CBD population, it was important to determine whether the bottom trawl fishery was over-utilizing the benthic habitat. Finally, CBD diet-based methodologies and analysis of the composition and trophic level of the fish catch were employed in order to explore the existence and the degree of competition. Common Bottlenose Dolphin Population. Materials and Methods The source of information was based on half-day dedicated coastal surveys which took place sporadically between 1998 -2002 and systematically from 2003 to 2007., Navigational and weather condition data were collected throughout the surveys and within the sighting, group size and composition as well as behavioral and photo-identification materials were recorded. Results and conclusions A total of 232 surveys were performed between 1998 and 2007, covering over 3,000 km of trackline, along the central Israeli coastline. CBD was the only species sighted. The overall encounter rate increased significantly when searching around bottom trawlers . The coastal CBD population prefers depths >40m; the bottom trawl fleet works mostly between the 35-55 m depth contours. Most births occur during the warm months. Mean group size was 5.7 ± 6.9, significantly larger in spring (7.5) than in summer (3.4).The Sighting frequency was independent of season, suggesting a year-round and year to year stability of population size in the study area, estimated at 360 individuals. As for composition, there appears to be a small resident nucleus in the study area, of around 20 animals out of 155 individually identified animals. The cumulative discovery curve is still steadily increasing, suggesting an 'open' population. Sighted groups were mainly engaged in foraging behavior, series of long dives interrupted by short periods of ventilation at the surface, either while following bottom trawlers or without much horizontal movement. Of 23 dolphins sighted four times or more, all were observed at least once foraging behind a bottom trawler, suggesting behavior common to all members of the resident population, rather than a specialty of some members, as described in other parts of the world. Bottom Trawl Fishery Materials and methods Sources of information for assessing the bottom-trawl fishery effort and catch trends were the annual reports by the Israeli Department of Fisheries for the years 1949-2006. The author was involved in the collection of data for the annual publications of 2004-2006. Results and conclusions Fishing effort showed an overall increasing trend from 1949-2006, with a transient decrease in the sixties. Effort is not the sole determinant of the catch, fishing efficiency is another. New technologies such as radar, sonar, satellite navigation tools (GPS) have been introduced to enhance the efficiency and these had kept on improving during the six decades of data collection. Also, stronger engines and propeller nozzles improved the towing capabilities of the boats, and new fishing nets with larger vertical opening and better rigging had been introduced. When analyzing the time-trend of the overall annual fished biomass (catch) in units of kg per one fishing day per boat (Catch per Unit Effort - CPUE), the effect of the increased efficiency does not become evident and should be kept in mind. Until the mid fifties, the CPUE showed a significant increase and since then, the trend has reversed. The data from the fifties also shows that CPUE was effort-independent, suggesting that fish abundance did not set catch limits, thus allowing an increase of the fleet without affecting CPUE. In the sixties, seventies and the beginning of the eighties, effort and CPUE were constant, suggesting a possible equilibrium between the fishery and the fish stock. This, however, could have been only a fictitious stability, with improvement of technology and use of new fishing grounds (e.g. north Sinai) compensating for over-fishing and dwindling stocks. In the late eighties and the nineties, the effort had increased significantly over that of the sixties to mid eighties, but the CPUE had decreased. Mullidae (goatfish) is a bottom-dwelling fish family, which had been a major target family for the local bottom trawl fishery during these years .The graph of CPUE against time for the Mullidae shows a decreasing trend, very similar to that of the overall catch. The decreasing time-trend was demonstrated for most bottom-dwelling commercial fish families. Penaeidae (shrimps), currently the second most important family in the gross income of the bottom trawl catch, shows an increasing time-trend of the CPUE. This might be a case of disturbance-tolerant species, for which bottom trawling creates new habitats, while their relatively short life cycle enables them to recruit in the face of growing fishing effort. Fishing down the marine food web is a worldwide phenomenon in which the composition of the catch is shifted away from predators (high trophic level) to plankton-eaters (low trophic level); a basic assessment for the Israeli bottom trawl fishery catch data suggests a similar trend, even though there is a significant increase with time of the high trophic level catch. The latter could be the result of increases in the CPUE of high trophic level families, such as the Sphyraenidae. One explanation for this increase is the higher vertical opening of the nets and the stronger engines which have enabled the fishers to improve their catch on this semi-pelagic fast swimming species. Another reason may be the regional proliferation of the Lessepsian migrant, Sphyraena chrysotaenia, which did not oust the local species, but has become an important component of the catch. Comparing CBD Diet to Bottom-Trawl Catch Materials and methods The data-base included carcasses of beached and/or by-caught CBD, 7-8 such cases are available annually, on average. Twenty three animals had stomach contents which were pooled for the comparison to the bottom trawl catch. Muscle samples of CBD and of commercial fish and invertebrates were analyzed for stable isotopes (d15N and δ13C). Results and conclusions The diet of CBD along the Israeli coastline was mainly composed of fish; cephalopod prey was less important and shrimps remains were not found. These findings matched results from the Western Mediterranean Sea. The estimated annual food consumption of the local CBD population was found to be very similar to the annual bottom trawl catch, setting the ground for potential competition. Yet, when comparing the proportion of different fish families in the CBD stomach contents to the local trawl-fishery catch, Sparidae (sea breams) was the only family showing equal frequencies between the potential competitors. Most members of this family have a relatively low commercial value. Also, the most prevalent prey item of CBD was the Balearic conger Ariosoma balearicum, a non-commercially important fish. The overall comparison suggests that the local CBD population and the local bottom-trawl fishery fleet do not target the same items and as such, are not in direct competition. The stable isotope analysis has shown a similar trend. When subtracting the estimated trophic enrichment factor from the δ15N value of the CBD, the resulting δ15N value turns out to be lower than that of most commercial species, again suggesting that the local CBD population and the local bottom trawl fishery fleet are actually exploiting different levels of the food-web. General Discussion The CBD occurs in a wide variety of habitats worldwide. The existence of several populations in near-shore areas, where they are relatively easily accessible for researchers, makes the CBD the best-studied cetacean. However, there are a limited number of long term researches on CBD in the Mediterranean Sea. This dissertation is the first step in establishing a long-term study on the CBD in the easternmost Mediterranean Sea - a population living in ultra oligotrophic waters, and showing a perturbing dependence on the local bottom-trawl fleet. On the one hand, the CBD obtains an easy meal from the bottom trawl net; on the other hand, over-exploitation of the benthic resources harms the CBD directly and indirectly, and by-catch in the trawl-net is a major cause of death to the local population. Is the practice of foraging around and inside the net just a manifestation of an intelligent predator taking advantage of an easily accessible source of food or is the population motivated by a necessity arising from the scarcity of food in its natural marine environment? Their intelligence is uncontested; their ability to adapt is well documented as well. Therefore it is reasonable to believe that they have learned to consume these easy meals. However, some facts, emerging from the present research, hint at nutritional stress. The local CBD population forages most of the day, similarly to a population studied in the Adriatic Sea, were food limitation was suggested, and unlike CBD populations studied in the UK, USA and Australia. Skinny dolphins are frequently observed in the study area. This is seemingly mainly related to the lesser insulatory demands during the warm period, with a resultant decrease in blubber thickness to allow cooling of the body. However, judged by lower catch rates during the warm period, the contribution of nutritional stress, accentuating the thermoregulatory effect, cannot be ruled out. __________________________________________________________________ Aviad Scheinin, Ph.D <mailto:schei...@research.haifa.ac.il> schei...@013.net.il <http://immrac.haifa.ac.il/> http://immrac.haifa.ac.il/ Chairman, IMMRAC - Israel Marine Mammal Research & Assistance center Home address: Tirat Shalom, P.B. 1356, Nes-Ziona 74052, Israel Tel 972-8-9406584 Mobile 052-3571193 __________________________________________________________________
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