[MARMAM] PhD Thesis on the Israeli bottlenose dolphins and their interaction with bottom trawlers

scheinin scheinin at 013.net
Mon Jan 25 12:07:33 PST 2010


 

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
scheinin at 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:scheinin at research.haifa.ac.il> scheinin at 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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