Based on fishermen logbooks, this study assessed turtle captures in Bahrain's shrimp trawl fishery for five fishing seasons during 1998–2003. Estimated total turtle captures were 298 ±184, 264 ±171, 433 ± 243, 394 ± 227, and 234 ± 177 cases which were caught during 39,147 ± 1,269, 35,761 ± 12,400, 42,747 ± 13,637, 37,071 ± 11,781, 43,923 ± 11,994 fishing days, respectively. It is likely that the green turtle was the species involved for most of these turtle captures. Fishing effort mean values were significantly different between seasons and fishing areas; while turtle capture rate mean values were not significantly different between seasons, months and areas. This suggests that turtles are randomly appearing in the shrimp fishing areas irrespective of the month, season, or fishing ground. These turtles are likely foraging in parts of the shrimp fishing ground or passing through the area during their migrations. Shrimp fishing grounds have previously been identified as a foraging habitat for sea turtles. Conservation actions are required to minimize the threat of shrimp trawl nets on green turtles. An immediate action should be to deliver a fishermen's awareness program that would improve their handling of live marine turtles once caught in the nets. Furthermore, fishermen should be required to adopt gear modifications and the government should enforce regulations that minimize the impact of shrimp trawls on Bahrain's green turtle population. These requirements are not only a national necessity, but contribute to the achievement of global conservation objectives.
Shrimp fishing grounds have been identified as a foraging habitat for sea turtles (Hildebrand, 1983; Miller, et al., 1989; Epperly, et al., 1995). In the northern Arabian Gulf, green turtle nesting areas are restricted to Karan, Kurayn, Jana, Jurayd, and Harqus islands found off the Saudi coast; while no nesting activities were observed along the Bahrain coast (Miller, 1989).
Bahrain's shrimp fishery targets single penaeid shrimp, i.e. Penaeus semisulcatus. This species prefers grounds with a high mud content (Somers, 1994); in Bahrain's waters, shrimp fishing grounds are found in areas where the average particle size is less than 400 μ in depth, ranging from 2 to 20 m (Abdulqader, 1995). Shrimp fishing grounds vary in their total areas and are distributed to the East and North-east of Bahrain. These fishing grounds are identified by their local names (Figure 1) (Abdulqader, 2006).
The number of boats involved in shrimp fishing has increased dramatically since the mid-1980s, from 40 boats to about 400 boats during the 1998/99 shrimp fishing season (Abdulqader, 2001). The increased number of boats involved in shrimp fishing has caused an over-fishing problem and increased the bycatch, thereby threatening wildlife species accidentally caught in shrimp trawls, particularly marine turtles. Based on fishermen logbooks, this paper assesses turtle capture in shrimp trawls during five shrimp seasons; 1998–1999, 1999–2000, 2000–2001, 2001–2002, and 2002–2003.
The fishing log scheme was introduced in 1985 to collect shrimp trawl data (Abdulqader, 1988) for all shrimp boats working in Bahrain's waters. The submission of fisheries data is mandatory for Bahrain's fishing license holders, including shrimp fishermen. The logbook presents the fishing activities of a boat on a daily basis; information recorded includes the fishing date, name of fishing area, catch of shrimp and other important species, number of hauls, and average duration of the haul. In addition, records on the presence and number of sea turtles accidentally caught in the nets are also required to be noted on a daily basis. The shrimp season usually ranges from mid-July to mid-March with a ban period which usually extends for a 4-month period.
Materials and Methods
This study is based on the shrimp fishing monthly logbook data collected for the seasons: 1998–1999, 1999–2000, 2000–2001, 2001–2002, and 2002–2003.
Estimation of the total turtle captures (I) and fishing effort (E) for (i) month and (j) fishing ground is based on three components: (1) number of working boats (Nwi) during the respective month, (2) at boat level, effort and capture mean values for respective months and areas, (3) number of boats which fished in the respective areas during the months.
The reported boats made up 71 to 88% of total active boats. Boats that submitted correct reports (n) were considered as a sample representing the whole fishing fleet. The ratio of working to non-working boats provided by the correct reports was used for the estimation of the total number working boats (Nwi). The calculation procedure is expressed empirically by Equation (1).
Nwiis the number of working boats during i month,
nwi, nnwi is the number of working and non-working boats in the sample,
and N is the number of active boats.
The fishing fleet distribution among the fishing grounds was generally governed by the catch rates. This usually caused uneven distribution of the fishing fleet across the fishing areas. The correct reports provided the total number of fishing days spent in the different fishing areas. The percentages of these values (dij) were be taken to represent the distribution of fishing fleets in different areas for the given month. The calculation procedure for both turtle capture and effort for a given month and given fishing area is expressed empirically by Equations (2) and (3).
Two fishing effort units can be used; the number of fishing days, and the number of fishing hours. The latter effort unit is the product of multiplying fishing days by the average number of tows per day by the average tow duration. The turtle capture rates were calculated by dividing the total captures by effort for the respective month and the fishing grounds.
The numbers of licensed (active) boats for these seasons were 403, 388, 403, 394 and 376 boats, respectively (Directorate General for the Protection of Marine Resources). The number of reported boats ranged from 275 to 357 (Table 1); however, the monthly means of numbers of reported boats were not significantly different as indicated by the ANOVA test (F = 0.475 at p = 0.864). The ANOVA test indicated significant differences between the means for reported boat numbers in season (F = 18.347, p < 0.0001), but the non-homogeneity of the variance as indicted by Levene Homogeneity test (3.221, p = 0.024) invalidated differences between means) Sokal and Rohlf, 1981).
Effort and tow duration for months, seasons, and fishing areas
The Univariate Analysis of Variance test was applied to determine if there were statistical differences in effort mean values for different months, seasons, and fishing areas. The test indicated significant differences in the effort means between seasons and fishing areas for both effort units (Table 2). Effort mean values and their confidence intervals (95% CI) for different months and fishing areas are presented in Figures 2 and 3. Effort mean values were highest in the case of “Qumais” and “Umudood” areas, which are significantly different from most of the means for the other areas. The effort mean value for the 2002–2003 season is significantly different from the 1998–1999 and 1999–2000 seasons (Figure 3).
The Univariate ANOVA (Table 3) suggests significant differences in haul duration means between fishing areas, while it shows insignificant differences between the different months and seasons. For fishing areas, the error bar plots (Figure 4) show that the highest tow duration mean value was slightly above 2 hours in the “Umudood” area which is the biggest shrimp fishing area in Bahrain's coastal waters (Figure 1). Shorter tow durations were found in smaller fishing areas: e.g. Tubli, Aljarim, Mina Sulman (Figures 1 and 4).
Turtle capture rate mean values (in days) and their 95% CI (Figures. 5–7 for different months, seasons, and fishing areas) suggested insignificant differences between the catch rate mean values for different months, seasons, and fishing areas.
Total turtle captures for the five shrimp fishing seasons from 1998–1999 to 2000–2003 ranged from 234 (in 2002–2003 season) to 433 captures (in 2000–2001 season) (Table 4). These estimates have high confidence interval (95%) values indicating insignificant difference between these estimates, as indicated by Figure 6. Total effort estimates also show high confidence interval values which ranged from about 36,000 days in the 1999–2000 season to about 44,000 days in the 2002–2003 season (Table 4).
The highest turtle capture rates were consistently found in “Qumais” throughout the five fishing seasons (Figure 8). Both “Qumais” and “Umudood” areas received the major share of the fishing effort, while the turtle captures in the latter area were low and inconsistent for different seasons (Figures 8 and 9). High turtle captures were seen in the case of “Alsheikh” fishing area during the 2001–2002 season, and for “Jaw” area during the 2000–2001 season (Figure 8). These areas were located within the shrimp fishing areas found south of “Fasht Al-Adhom” which also includes the “Qumais” area which had the highest number of turtle captures (Figures 1 and 8). Several shrimp fishing areas did not show turtle captures, these included “Tubli Bay”, “Umalnaisan”, “Tugailib”, “Aljarim”, and “Khur Fasht”.
Despite the fact that the “Qumais” area had the highest number of turtle captures this area showed low capture rates (Figure 10). The highest turtle capture rates were found in “Mishtan”, Ras Albar, and “Debil” areas during the 1998–99 season. From 2002–2003, a high capture rate was found in the “Bouys” area (Figure 10).
This study provided estimates of total turtle captures in shrimp trawls for five shrimp seasons; 1998–1999 to 2002–2003. Although estimates are not provided species-wise, it is likely that the green turtle is the species involved for most of these turtle captures. Two species of marine turtles inhabit waters around Bahrain: the green turtle and hawksbill turtle (Miller and Abdulqader, 2009). Miller (1989) found that green turtles are most abundant; he suggested a ratio of one hawksbill to 23 green turtle.
Significant differences were found in mean fishing effort values between seasons and fishing areas (Table 2); while the mean values for turtle capture rates were not significantly different between the seasons, months and areas. This suggests that turtles were randomly appearing in the shrimp fishing areas irrespective of the month, season, or fishing ground. These turtles were likely foraging parts of the shrimp fishing ground or, possibly passing through the area during their breeding migrations.
Most turtle captures occurred in the “Qumais” area located south of Fasht Al-Adhom (Figure 1). This area and the northern “Umudood” area received most of the fishing effort throughout the five seasons. This suggested that turtle abundance in the southern ground of “Qumais” may be higher than the “Umudood” area, located in the north (Figure 1). If so, this would most likely be the result of more intensive sea-grass beds in these areas (Al-Zayani, 2003). On the other hand, it is possible that a higher fishing effort (Figure 2) might have increased turtle captures in this same area. Furthermore, the Dead Marine Turtle Monitoring Program observed that most of the turtle mortalities occurred in the northern areas (in “Aljarim” and “Slaesel” areas in Figure 1) (Abdulqader, 2008). This suggests that higher turtle abundances might found in the northern areas, including “Umudood”, the biggest fishing area (Figure 1) or that longer tow duration contributes to turtle mortalities in the northern areas (Figure 4). Interestingly high turtle mortality did not indicate high abundance in this case.
Tow duration in Bahrain's waters varied with the fishing area (Figure 4), where higher durations were found in the northern areas, with the maximum mean duration being slightly above 2 hours in the largest fishing areas. In the northern Australian fishery, typical shrimp tow duration extended to 3 hours in the tiger prawn fishery, where a 21% delayed mortality was determined in the case of the green turtle (Poiner and Harris, 1996). Tow duration has considerable impact on the turtle's survival: increasing tow times result in a rapid escalation in mortality (10–200 min in summer and 10–150 min in winter), and eventually reach a plateau of high mortality (Sasso and Epperly, 2006).
Total captures estimated by this study for the seasons 1998–1999 to 2002–2003 were 298, 264, 433, 394 and 234 cases caught during 39147, 35671, 42747, 37071 and 43923 days, respectively. In the northern Australian prawn fishery, 5,503 and 5,238 turtles were caught during 27,049 and 25,746 fishing days in 1989 and 1990, respectively (Poiner and Harris, 1996). This suggests lower turtle capture rates are found in the Bahrain's waters compared to the northern Australian shrimp fishery.
The Dead Marine Turtle Monitoring Program estimated that 170 turtles died in 2007 (Abdulqader, 2008); most of these mortalities were green turtles and were due to shrimp trawl nets. This suggests that a considerable percentage of marine turtles caught in shrimp trawls eventually die. The shrimp fishery of the southeast United States was estimated to have caused more sea turtle mortalities than that of all other human activities combined (National Research Council, 1990). Conservation actions are required to minimize the threat of shrimp trawl nets on green turtles. To prevent further mortalities in Bahrain's turtle population, an immediate action should be to develop and deliver fishermen awareness programs on how to improve handling of live marine turtles once caught in the nets. Furthermore, fishermen should be reqiured to adopt gear modifications to reduce captures such as Turtle Excluder Devices (TEDs) modified for local conditions. Further, enforce regulations that minimize the impact of shrimp trawls on Bahrain's green turtle population should increase. These requirements are not only a national demand, they are needed to contribute to achieving global conservation as green turtles are classified as a globally endangered species (IUCN, 2003). Further, gillnets have the potential to threaten the marine turtle population (Cheng et al., 1997). The use of drift gillnets is prohibited in Bahrain's water but a considerable amount of fishing by this method is still illegally conducted. Further work is required to document the interaction of gillnet fishing with the turtle population.
This study estimated that 234 to 433 turtles were captured during the five studied seasons. The appearances of these turtles in shrimp fishing grounds were found to be randomly occurring, irrespective of the month, season, or fishing ground. Longer haul durations spent in the northern grounds likely will cause higher mortalities in these grounds.
The captures of marine turtles in shrimp trawl are evidence that Bahrain's shrimp trawlers are threatening to these creatures. Shrimp grounds are extending into sea-grass areas, which are feeding grounds for green turtles, the most common species in Bahrain waters. It is recommended that the impact of Bahrain shrimp trawl fishery on marine turtles be minimized by implementing gear modifications and regulations, particularly the introduction of turtle excluding devices.
This study could not have been achieved without the support of Mr. Jassim Al Qaseer, the Director General for the Protection of Marine Resources, who gave permission to access the fishermen's monthly logbooks. Thanks go to all fishermen who submitted correct reports. Thanks also to Mr. Ahmed Al-Sheikh and Mr. Ali Shuiab for their work on sorting the logbook sheets and for data entry.