Abundance of Olive Ridleys Along Odisha Coast - Seaturtle.org

Chapter 20

Abundance of Olive Ridleys Along Odisha Coast: Sources of Mortality and Relative Importance of Fisheries Impacts K. Behera Subrata, C. S. Kar, Behera Satyaranjan, John Sajan, K. Sivakumar and B. C. Choudhury

Abstract Despite international and national protection, sea turtles continue to be caught incidentally in large numbers along the coastal waters of Odisha. This chapter examines the mortality of sea turtles in three arribada site of Odisha, focusing on offshore distribution and number of carcasses of olive ridley found along the three zones. A total of 14,035 and 3,481 turtles were washed ashore dead along the three important arribada sites of Odisha in two consecutive study period 2008–2009 and 2009–2010 due to trawling activity. We compared the number of turtles stranded on beaches adjacent to the fishing grounds with the near shore fishing activity within 6 km from the coast and at a depth of 30 m. The objective was to correlate the beach stranding with degree of fishing activities and to evaluate beach stranding as an indicator of fishery-induced mortality. The mortality and trawling activity was further classified into pre-mating, mating, and postmating seasons for comparison. There was a positive significant correlation with the seasons in trawling and mortality of turtles. The findings conclude that turtle stranding during the mating period is highest in the turtle abundance zones in comparison to adjoining areas. In places where near shore trawling was more mortality of turtles and where the fishing activities were moderate or low the turtle mortalities was found to be less. The high trawling near shore coincide with the mating period of turtles due to which large number of mortality occurs. Keywords Olive ridley Mortality

Trawl intensity Line transect Relative abundance

K. B. Subrata (&) C. S. Kar B. Satyaranjan J. Sajan K. Sivakumar B. C. Choudhury Wildlife Institute of India, Chandrabani, #18Dehradun, Uttrakhanda, India e-mail: [email protected]

K. Venkataraman et al. (eds.), Ecology and Conservation of Tropical Marine Faunal Communities, DOI: 10.1007/978-3-642-38200-0_20, Ó Springer-Verlag Berlin Heidelberg 2013

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20.1 Introduction Odisha offshore water is a well-known breeding ground for olive ridleys. Along 480 km of the coastline of Odisha there are three identified arribada nesting beaches for olive ridleys. Toward north of Odisha Gahirmatha, central is Devi and in extreme south is Rushikulya. During the past four decades, Odisha ridleys came into limelight due to large-scale mortality. This is due to by catch in trawl fisheries which has been determined to be a major source of mortality of adult sea turtles (James et al. 1989; Kar 1980; Pandav et al. 1997, 1998; Gopi et al. 2002). An estimated number of 5,000–15,000 sea turtles died each year in trawling activities which is more than any other human-induced source of mortality (Gopi et al. 2002; Shankar and Mohanty 1999). The influence of trawling on sea turtle survival is especially noteworthy in the Odisha coast where most of the trawling activities are from October to May and most trawling activity is concentrated close to shore (Gopi et al. 2002). Sea turtles are also found close to shore during the breeding season (Hildebrand 1983; Byles 1989; Lohoefener et al. 1990; Manzella and Williams 1992; Tripathy 2004; Ram and Pandav 2004). Trawler numbers have increased more than four times from 1980 to 2005 and trawling activity have multiplied in the offshore water of Odisha. This is one of the major reason that large number of olive ridleys are washed ashore dead every year due to heavy near shore trawling locally and it is difficult to correlate the mortality of turtle due to fisheries interaction as evidences are rarely identified. Assessing the mortality of oceanic species is very difficult. Shoreline surveys for stranded carcasses can provide mortality data for species that frequently come to near shore waters, but these severely underestimate overall mortality because (1) surveys are usually limited to a small fraction of coastlines and (2) the majority of turtles that die at sea either naturally or due to by catch do not strand (Epperly et al. 1996; Hart et al. 2006; Leeney et al. 2008; Tomás et al. 2008). This study deals with monthly sea turtle stranding along shoreline vis-a-vis trawl fishing effort in coastal waters (Figs. 20.1 and 20.2). Trawl fishing effort is reported as the days fished within spatial units represented for 30 m depth zone. To test the null hypothesis, we paired bimonthly stranding along segments of shoreline with bi-monthly trawling effort within 30 m depth intervals in the adjacent offshore waters. This was done because it was expected that beyond offshore the trawl activity took place, the less likely sea turtles impacted by such trawl activity would reach the shoreline, due to combined effects of surface currents, winds, waves, tides and decomposition of turtle carcasses (Murphy and Hopkins-Murphy 1989; Shoop and Kenney 1992). Also, it is possible that temporal-spatial distributions of sea turtles and trawl fishing activities overlap only within certain depth intervals (Magnuson et al. 1990).

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Fig. 20.1 a Mortality of Olive ridley turtles along Odisha coast. b Turtles trapped in the trawl net in the offshore waters off Odisha Coast

20.1.1 Study Area The study area was the coastline and coastal waters of Odisha. It was divided into three zones (Fig. 20.3), determined by taking into consideration the arribada sites along the Odisha coast. This study was made using depth contours of 30-m for sea turtle densities and trawl intensity were also estimated accordingly.

20.2 Methods The monthly sea turtle strandings and trawl fishing efforts were made in three zones: the upper coast (Gahirmatha), central coast (Devi) and lower coast (Rushikulya).

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Fig. 20.2 a Turtle trapped in the monofilament net in the offshore water of Odisha Coast. b Freshly washed ashore dead turtles predated by feral dog

20.2.1 Trawling Intensity Offshore trawling intensities were estimated for the 5 km area from the shoreline (0–30 m depth) of the three zones. In this study, we used linear correlation analysis to test the null hypothesis that there was no relationship between monthly sea turtle standings and trawl fishing effort in Odisha coast during 2008–2010. Sea turtles would not be captured in trawls if the temporal-spatial distributions of sea turtles and trawl fishing effort did not overlap to some extent. However, we have no prior reason to expect that temporal-spatial distributions of sea turtles and trawl fishing effort match exactly. Trawling in the Odisha coast varies seasonally and spatially as related to the annual cycle of occurrence and abundance of fish catch. It is most intense during the main fishing season, i.e, October–May.

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Fig. 20.3 Mass nesting rookery and Sporadic nesting beaches along Odisha coast

20.2.2 Sea Turtle Relative Abundance The encounter rates were estimated by line transect method in all the three zones within the same timeframe of the day for each month during the breeding period (November 2008–April 2009 and November 2009–April 2010) to assess the relative abundance of sea turtles. All the three zones have six transects and were equidistant (1 km) from each other.

20.2.3 Mortality Monitoring was carried out all along the three specified beaches to enumerate the number of washed ashore dead turtles in all the three zones on a daily basis. The length of Gahirmatha beach was 35 km, Devi 25 km and Rushikulya 20 km.

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20.3 Results 20.3.1 Sea Turtle Relative Abundance The relative abundance of sea turtles sighted during the surveys varied within three zones along the Odisha coast (Fig. 20.4). In general, the relative abundance of the Olive ridley sea turtles sighted was higher towards the south of Odisha coast (Rushikulya) than in the north (Gahirmatha) during 2008–2010. Overall densities remained high during the study period, (7.6 turtles/km) in Rushikulya region of Odisha during 2008–2009 and (10.2 turtles/km) during 2009–2010. Along the Central Odisha coast (Devi), the sea turtle abundance was observed to be extremely low (0.6 turtles/km) and (1.1 turtles/km) during 2008–2009 and 2009–2010 offshore sighting

trawls

30

November-December

January-Feburary

Rushikulya

Devi

Gahirmatha

Rushikulya

Devi

Gahirmatha

Rushikulya

0

Devi

15

Gahirmatha

percent in total

45

March-April

(2008-09) offshore sighting

trawls

40 30 20

November-December

January-Feburary

Rushikulya

Devi

Gahirmatha

Rushikulya

Devi

Gahirmatha

Gahirmatha

0

Rushikulya

10

Devi

percent in total

50

March-April

(2009-10) Fig. 20.4 Abundance of turtles along the three zones of Odisha coast and the trawl intensity along adjacent coastal waters (bimonthly data from 2008 to 2009 and 2009 to 2010)

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seasons, respectively. The overall percentage of sightings recoded during 2008– 2009 and 2009–2010 was highest during the month of January–February in all the three zones, Gahirmatha 23.21 and 32.33 %; Devi 1.82 and 1.93 % and Rushikulya 35.5 and 42.7 %, respectively. This is due to mating activity that was in peak during these two months (Fig. 20.4).

20.3.2 Trawling Intensity Trawling intensity in near shore coastal waters of Odisha coast over the two breeding seasons showed a very distinct pattern (Figs. 20.5 and 20.6). The intensity of near shore trawling was extremely high in northern part of Odisha; Gahirmatha during 2008–2009 and during 2009–2010 in central Odisha in comparison to other three zones. Towards the south, near Rushikulya trawling intensity was minimal during the survey period due to the fact that there are no local trawlers in this area. This is again due to the fact that there are no big rivers draining into the Bay of Bengal in this area. Gahirmatha had the highest trawling intensity, i.e. 1.15 trawlers/km/day during the month of February 2008–2009. During 2009–2010 seasons, however, Devi zone had the highest trawling intensity

Gahirmatha

Devi

1.20

trawlers/km/day

Fig. 20.5 Trawling activity in near shore coastal waters at a depth contour of 30 m at two zone during 2008–2009. The third zone (Rushikulya) was not included as no trawling activity was recorded within 30 m contour at this site

1.00 0.80 0.60 0.40 0.20 0.00 November December January

1.60

Gahirmatha

March

April

March

April

Devi

1.40

trawler/km/day

Fig. 20.6 Trawling activity in near shore water at a depth contour of 30 m at two zone during 2009–2010. The third zone (Rushikulya) was not included as no trawling activity was recorded within 30 m contour at this site

February

1.20 1.00 0.80 0.60 0.40 0.20 0.00 November December January

February

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trawls

40

November-December

January-Feburary

Rushikulya

Devi

Gahirmatha

Rushikulya

Devi

Gahirmatha

Rushikulya

0

Devi

20

Gahirmatha

percent in total

60

March-April

Fig. 20.7 Seasonal changes in sea turtle stranding on sea beaches and Trawl—fishing effort in the coastal waters at three locations along the Odisha coast for 2008–2009. The three areas differ greatly in the abundance of turtles sighted by line transect surveys and trawls fishing effort. Rushikulya rookery had the fewest turtle’s mortality for least trawls fishing, followed by Devi, and Gahirmatha rookery; the largest number of dead turtles for trawls fishing effort

of 1.33 trawlers/km/day during the month of February. In order to compare the seasonal change in the pattern of trawling, it was further classified into three seasons such as pre-mating period (November–December), mating period (January–February) and post-mating period (March–April). Between the seasons the near shore trawling was highest during the mating period (January–February) (Figs. 20.7 and 20.8).

mortality

trawls

20

November-December

January-Feburary

Rushikulya

Devi

Gahirmatha

Rushikulya

Devi

Devi

Gahirmatha

0

Gahirmatha

10

Rushikulya

percent in total

30

March-April

Fig. 20.8 Seasonal changes in sea turtle stranding on sea beaches and Trawl—fishing effort in the coastal waters at three locations along the Odisha coast for 2009–2010. The three areas differ greatly in the abundance of turtles sighted by line transect surveys and trawl fishing efforts. Rushikulya rookery has minimum turtle mortality followed by Gahirmatha and Devi. The highest mortality was recorded along Devi coast

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20.3.3 Mortality (Washed Ashore Turtles) All the dead turtles counted during the survey were adult olive ridley. A total of 14,035 turtles were counted during 2008–2009 and 3,481 turtles during 2009– 2010. Highest (59.9 %) mortality recorded at Gahirmatha during 2009–2010 during the mating period (January–February) and 29.2 % was at Devi during the same season for 2009–2010. The overall percentage of mortality was observed to be highest during mating period (January–February) in all the three zones. Rushikulya have the least mortality in comparison to other two zones as a result of no near shore trawling. During 2008–2009, a total 54 mortality was recorded and in 2009–2010 it was 59 in number which is least in comparison to other two zones.

20.3.4 Trawling and Mortality On the two coasts (Gahirmatha and Devi), the correlation coefficient ‘r’ were positive with the seasons (r = 0.80 and r = 0.91 during 2008–2009 and 2009– 2010) in respective years. These correlations indicated that sea turtle stranding increased as fishing efforts increased in the coastal waters within 30 m depth counter during the two seasons 2008–2009 and 2009–2010.

20.4 Discussion Comparisons of the spatial distribution of trawl intensity to the distribution of sea turtles provide an insight into potential period of intensive interactions (Fig. 20.4). The positive correlation of presence of olive ridley sea turtles and trawling intensity distribution this way as per the findings may be related to the food and feeding habit of the sea turtles. Sea turtle distribution is most probably a result of several factors. Sea turtles could be congregating along the Odisha coastal waters in such huge congregations because the habitat as well as environmental conditions is more suitable. Sea turtles mating and nesting activity takes place in various habitats, but primary mating areas are often located on relatively shallow shelves and near large river mouth areas. The study also clearly identified a relatively high abundance of sea turtles and low near shore fishing intensity along the coastal waters of south Odisha (Fig. 20.4). Restricting fishing effort in the areas of high trawling and high turtle abundance would protect the turtle abundance in turtle congregation zones in northern and in central Odisha. Furthermore, the state government have already considered seasonal closures for the trawling activity in near shore coastal waters of Odisha coast from November to May in the known turtle congregation zones which is 20 km zone from the coast line in three designated areas. But this

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regulation is not always implemented in proper manner due to which sea turtles mortality occurs through out breeding period. This study has focused on the relative spatial pattern of abundance of ridley sea turtles along the Odisha coast especially at the three zones (arribada beaches) as well as on the potentially intense interactions between trawling activities and sea turtles. The surveys were conducted during November–April, in two seasons which are the months of highest fishing activity. Comparisons were made between trawling intensity to verify the regions of potential sea turtle and trawler interactions and also to have an insight about how sea turtles respond to intense fishing pressure. The analysis provides an overlap of trawling intensity and sea turtle abundance from December to April and mainly during the breeding season when turtles generally remain in the near shore shallow waters of the coastal region.

References Byles RA (1989) Satellite telemetry of Kemp’s ridley sea turtle, Lepidochelys kempi. The Gulf of Mexico, pp 25–26. In: Eckert SA, Eckert KL, Richardson TH (eds) Proceedings of the Ninth Annual Workshop on Sea Turtle Conservation and Biology. NOAA Technical Memorandum National Marine Fisheries Service-SEFC-232 Epperly SP, Braun J, Chester AJ, Cross FA, Merriner JV, Tester PA, Churchill JH (1996) Beach strandings as an indicator of at-sea mortality of sea turtles. Bull Mar Sci 59:289–297 Gopi GV et al (2002) A quantitative analysis of incidental capture and mortalities of sea turtles during commercial shrimp trawling using Turtle Excluder Devise (TED) along the coastal waters of Orissa. Wildlife Institute of India, Dehradun, India pp 1–57 Hart KM, Mooreside P, Crowder L (2006) Interpreting the spatio-temporal patterns of sea turtle strandings: going with the flow. Biol Conserv 129:283–290 Hildebrand HH (1983) Random notes on sea turtles in the western Gulf of Mexico, pp 34–40. In: Owens DW, Crowell D, Dienderg G, Grassman M, McCain S, Morris Y, Schwantes N, Wibbels T (eds). Western Gulf of Mexico Sea Turtle Workshop Proceedings. Texas A&M University Publication TAMU-Sg-84-105 James PSBR, Rajagopalan M, Dan SS, Fernando AB, Selvaraj V (1989) On the mortality and stranding of marine Mammals and Turtles at Gahirmatha, Odisha from 1983 to 1987. J Marine Biol Assoc India 31(1&2):28–35 Kar CS (1980) The Gahirmatha turtle rookery along the coast of Odisha, India. IUCN/SSC Marine Turtle Newslestter, Canada 15:2–3 Leeney R, Amies R, Broderick AC, Witt MJ, Loveridge J, Doyle J, Godley BJ (2008) Spatiotemporal analysis of cetacean strandings and bycatch in a UK fisheries hotspot. Biodiv Conserv 17:2323–2338 Lohoefener R, Hoggard W, Mullin K, Roden C, Rogers C (1990) Association of sea turtles with petroleum platforms in the north-central Gulf of Mexico. OCS (Outer Continental Shelf) Study/MMS 90-0025. U.S. Department of the Interior, Minerals Management Service, Gulf of Mexico OCS Regional Office, New Orleans, Louisiana, USA Magnuson JJ, Bjorndal JA, DuPaul WD, Graham GL, Owens DW, Peterson CH, Pritchard PCH, Richardson JI, Saul GE, West CW (1990) Decline of sea turtles: causes and prevention. National Research Council. National Academy Press, Washington, D.C Manzella SA, Williams JA (1992) The distribution of Kemp’s ridley sea turtles (Lepidochelys kempi) along the Texas coast: an atlas. NOAA (National Oceanic and Atmospheric

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Administration) Technical Report, National Marine Fisheries Service 110, U.S. Department of Commerce, Seattle, Washington, USA Murphy TM, Hopkins-Murphy SR (1989) Sea turtle and shrimp fishing interactions: a summary and critique of relevant information. Center for Marine Conservation, Washington, D.C Pandav B, Choudhury BC, Kar CS (1997) Mortality of Olive Ridley turtles (Lepidochelys olivacea) due to incidental capture in fishing nets along the Odisha coast, India, Oryx 31(1):32–36 Pandav B, Choudhury BC, Shankar K (1998) The Olive Ridley sea turtle (Lepidochelys olivacea) in Odisha: An urgent call for an intensive and integrated conservation programme. Curr Sci 75:1323–1328 Ram K, Pandav B (2004) Studies on the offshore ecology of the olive ridley sea turtle (Lepidochelys olivacea) in the Gahirmatha marine sanctuary, Odisha, India. In: Proceedings of the Twenty-First Annual Sympoisum on Sea Turlte Biology and Conservation Shankar K, Mohanty B (1999) Operation Kachhapa: in search of a solution for the Olive Ridleys of Odisha. Mar Turt Newsl 86:1–3 Shoop CR, Kenney RD (1992) Seasonal distributions and abundances of loggerhead and leatherback sea turtles in waters of the northeastern United States. Herpetological Monogr 6:43–67 Tomás J, Gozalbes P, Raga JA, Godley BJ (2008) Fisheries bycatch of loggerhead sea turtles: insights from 14 years of strandings data. Endang Species Res. doi:10.3354/esr00116 Tripathy B (2004) A study of the offshore distribution of olive ridley turtles (Lepidochelys olivacea) in the coastal waters of Rushikulya rookery along the Odisha coast, India