Incidental captures of sea turtles in the driftnet and longline fisheries in northwestern Morocco

By Ana Zangroniz, Marine Conservation Student

One important issue in marine conservation lies with the preservation of a healthy sea turtle population. Of the seven species (Leatherback, Green, Loggerhead, Hawksbill, Olive Ridley, Kemps Ridley, and Flatback), six are endangered or threatened. Besides the fact that these creatures are visually stunning, they play a crucial role in marine ecosystems, which can directly affect human beings and our livelihoods. For example, green sea turtles feed on seagrass. This grazing keeps seagrass beds healthy, helping maintain critical habitats for many life stages of scallops and mollusks that humans depend upon as a food source (Carroll et al. 2012, Nizinski 2007). Additionally, when female sea turtles come ashore to lay their eggs, beach ecosystems are enriched, as turtle eggs are a significant source of nutrients for plant life (Vander Zanden et al. 2012).

Marine sea turtles have declined over the last few hundred years at an alarming rate, due to a variety of causes. Historically, turtles eggs were widely sought after as a major food source for human communities (Seminoff 2004). Fatalities are also caused inadvertently by human activity, particularly the consumption of plastic, which turtles may mistake for jellyfish, a common prey item (Sarti Martinez 2000). Turtles are also threatened by accidental capture in fishing gear, known as “bycatch.” This term refers to the portion of a commercial fishing catch that consists of marine animals trapped unintentionally. Many commercial fishing methods pose potential risks: both longline fisheries, which deploy hundreds or thousands of baited hooks, and driftnet fisheries which rely on large, indiscriminate nets, present serious threats to turtles. Based on their physiology and behavior, turtles can easily be hooked in longline fisheries or entangled in nets or lines. These entanglements render them unable to surface for air, and many eventually drown (Wang et al. 2007).

A leatherback sea turtle caught by a longline in Brazil. Photo credit: Guy Marcovaldi, Projecto Tamar Brazil, Marine Photobank

A research paper titled, “Incidental captures of sea turtles in the driftnet and longline fisheries of northwestern Morocco,” by Benhardouze et al. (2012) begins by identifying the main issue at hand: significant current threats to marine sea turtle populations. This study looks specifically at fishing based around the port of Tangier in Morocco. This area has large scale commercial fisheries, including tuna and swordfish (Benhardouze et al. 2012). The western end of Morocco borders the Straight of Gibraltar and is the migration corridor for loggerheads and leatherback turtles between the Atlantic and Mediterranean (Benhardouze et al. 2012). For this reason, interaction between fisheries and migrating sea turtles are almost inevitable. When doing background research on the frequency and severity of sea turtle bycatch in the area, Benhardouze and his coauthors discovered little, if any useful information and therefore undertook this research to ascertain the impact of commonly used fishing gear on marine sea turtles in the straights.

This study was conducted over two two-year periods, 2003-2004 and 2006-2007. No data was collected in 2005, for reasons unknown. The scientists themselves did not directly collect the data, instead relying upon reports compiled by fishermen. The data was generated as the fishermen went about their normal longline and driftnet fishing activities. Fishermen were asked to record all of instances of bycatch. Seven boats participated, with an average length of 15 meters, or about 50 feet. Data collection was broken down by fishing method into the following categories: driftnet catches and surface and bottom longline catches. Fishermen then recorded the number of turtles caught alive, dead, and comatose in each type of gear. Fishermen were taught to measure the curved carapace length (the length of the outer shell), allowing the researchers to approximate the age of the turtles caught. It was found that turtles were caught more frequently in driftnets than longlines. 73 turtles were caught during the years of the study, 64 loggerheads and nine leatherbacks (Benhardouze et al. 2012). Of the total catches, only four loggerheads had died. Using this catch data, it was possible for the researchers to generate hypothetical totals for fleet-wide bycatch during the study period. The yearly mean number of boats operating out of the Port of Tangier was 181, with an average of 9730.25 fishing days (Benhardouze et al. 2012). Using these figures, the scientists estimated that the entire fleet caught 719 loggerheads and 101 leatherbacks via driftnet during the four-year period. These numbers seem to indicate that the fleet is fairly large, however no exact numbers for the fleet are reported, nor do they indicate how many different fisheries are represented.

A positive outcome of the study was that the fishermen proved successful in collecting usable data for the scientists, which provided useful figures about the direct relationship between sea turtles and fisheries in the Port of Tangier region.  Given that the fishermen were on the lookout for turtles caught by nets and hooks, it is also possible that study participation led fishermen to release turtles more rapidly, potentially lowering mortality.  No data is available on post-release mortality rates.

Since the authors do not cite similar studies for comparison, it is difficult to ascertain whether or not the numbers obtained represent high or low figures of bycatch. In addition to this, there are some important questions to consider regarding the approach and methods of the study. For instance, the authors do not elaborate on the total size of the fishery that they collaborated with, nor do they mention whether the average boat length of participating fishermen is representative of the entire fleet.  Although there are logistical problems with engaging in large-scale research of this type, the weight which should be given to these findings depends strongly on the adequacy of these seven boats as a sample of the larger fishery. The authors do not specify how the participating boats were chosen, nor do they detail what compensation participating fishermen may have received. All of these factors could certainly skew the findings, resulting in data that might not be as faithful as desired.

Since the scientists were not able to be aboard for the data collection, the need for honesty and accuracy in the findings was a significant issue to be discussed with the fisherman. The paper states “after several meetings with the fishermen and their families participating in this study, a trust and understanding was established that facilitated data collection for this study.”  In spite of this, fishermen still may have had reservations about complete candor and feared repercussions if their number of total catches seemed high.

What do these discoveries tell? Driftnets appear to be the more significant culprit in high rates of turtle bycatch. Knowing this, perhaps turtle conservation groups can work with fishermen to find new solutions to this problem. One might be to investigate the implementation of units similar to the turtle excluder devices, used in modern trawl fisheries. Turtle excluder devices, or TEDs, are positioned in nets and divert turtles to an exit (Sala et al. 2011). Using this, scientists and conservationists will be better equipped to more specifically predict and pinpoint similar patterns in other high-traffic (turtle/fisheries) regions. The numbers can be brought to local governments, possibly opening the door for legislation further protecting turtles. Knowing what human behaviors pose the most significant threats to turtles is an important step in reducing those threats. Now that these findings have been published, the future for marine sea turtles may become a little bit brighter.

REFERENCES

Benhardouze W, Akisissou, M, Tiwari, M (2012) Incidental captures of sea turtles in the driftnet and longline fisheries in northwestern Morocco. Fisheries Research, Volumes 127-128, 125-132 Direct link: http://iiiprxy.library.miami.edu:2172/science/article/pii/S0165783612001300

Carroll, JM, Furman, BT, Tettlebach, ST, Peterson, BJ (2012) Balancing the edge effects budget: bay scallop settlement and lose along a seagrass edge. Ecology, Volume 93, Issue 7, 1637-1647

Nizinski, Martha (2007) Predation in subtropical soft-bottom systems: spiny lobster and molluscs in Florida Bay. Marine Ecology Progress Series, Volume 345, 185-197

Sala, A, Lucchetti, A, Affronte, M (2011) Effects of Turtle Excluder Devices on bycatch and discard reduction in the demersal fisheries of Mediterranean Sea. Aquatic Living Resources, Volume 23, Issue 2, 183-192

Sarti Martinez, AL (Marine Turtle Specialist Group) 2000. Dermochelys coriacea. In: IUCN 2012. IUCN Red List of Threatened Species. Version 2012.1. <www.iucnredlist.org>. Date accessed: 6 October 2012.

Seminoff, JA (Southwest Fisheries Science Center, U.S.) 2004. Chelonia mydas. In: IUCN 2012. IUCN Red List of Threatened Species. Version 2012.1. <www.iucnredlist.org>. Date accessed:  8 October 2012.

Vander Zanden, H, Bjorndal, K, Inglett, P (2012) Marine-derived nutrients from green turtle nests subsidize terrestrial beach ecosystems. Biotropica, Volume 44, Issue 3, 294-301

Wang, JH, Boles, LC, Higgins, B, Lohmann, KJ (2007) Behavioral responses of sea turtles to lightsticks used in longline fisheries. Animal Conservation, Volume 10, Issue 2, 176-182