YEAR 2022-23

Investigating the interaction between Irrawaddy dolphin (Orcella brevirostris) and Fisheries in the Eastern Gulf Coast of Thailand

Project Summary

The Irrawaddy dolphin (Orcella brevirostris) is a coastal and freshwater mammalian species found sparsely distributed throughout the tropical and subtropical Indo-Pacific (Jackson-Ricketts, 2016). Their  “Endangered” classification on the International Union for Conservation of Nature’s (IUCN) Red list (Minton et al., 2017) is largely from anthropogenic threats, particularly those relating to fisheries (Nelms et al., 2021).  In Thai waters, over exploitation of fish stocks is an ongoing issue causing reductions in fishery production & biomass (Wongrak et al., 2021), all of which can impact fish-eating mammals such as the O.brevirostris. To date little research has been done to quantify resource overlap between O.brevirostris prey and fisheries. Across a one-year period, this study will evaluate  prey  resource  and spatial overlap  between O.brevirostris sub  populations  in  the  Trat Province of Thailand, and fisheries. The results of which could support ecosystem-based fishery management in the future, with the intention of improving O.brevirostris populations as well as fishery catch yields.

Case for support

Part 1 – Literature Review

O.brevirostris are a cetacean species sparsely distributed across coastal waters of the subtropical and tropical Indo-Pacific (Jackson-Ricketts, 2016; Tubbs et al., 2020). As with other cetaceans they are considered important top down controllers of their marine environment primarily by the function of predator-prey relationships (Jackson-Ricketts, 2016; Ricci et al., 2021). Moreover, they have a role in the modification of benthic habitats as well as nutrient recycling and are considered essential players  in  a  marine  ecosystem’s  health  and  integrity  (Bowen,  1997;  Nelms  et  al.,  2021). O.brevirostris marine populations are preferentially distributed within shallower coastal waters in close proximity to shores & river mouths (Minton et al., 2011; Minton et al., 2017). This association is thought to be driven by freshwater input and prey distributions (Minton et al., 2017).

Currently listed as “Endangered” on the IUCN Red List of Threatened Species, the close association of the O.brevirostris to the coast has left them vulnerable to a host of anthropogenic issues including but not limited to; bycatch, shipping vessel collisions, and prey overfishing (Jackson-Ricketts, 2016; Parsons et al., 2015). For example, Minton et al. (2011) in the Kuching region of Malaysia saw 50% of O.brevirostris sightings within less than 50 metres of fishing vessels, increasing their risk of direct interactions  (Minton  et  al.,  2011).  Between  January  &  February  2013,  the  death  of  fourteen O.brevirostris as a result of fishing gear entanglement near Trat province in the Gulf of Thailand (Hines et al., 2015) further highlights this issue. Overlap between cetaceans and fisheries is not globally problematic; but rather restricted to certain regions and species (Kaschner & Pauly, 2005). The Eastern Gulf coast of Thailand is a problematic area for sub-populations of O.brevirostris, however to date little research has been done on the relative overlap (Nelms et al., 2021).

According to the  United  Nations  (UN)  Food and Agriculture Organisation, Thailand’s  Exclusive Economic zone (EEZ) comprises 41% of the marine catch within the Gulf of Thailand (FAO, 2013). Thailand’s fishing industry is commercially significant offering 2.4million tonnes to global fishery production and  keeping  almost  250,000  people  employed  annually  (Kulanujaree  et  al.,  2020). Commercially  significant   species  include  the   Indo-Pacific  mackerel  (Rastrelliger  spp.)  which encompasses around 41% of catches; as well as anchovies (Encrasicholina spp. and Stolephorus spp.), round scads (Decapterus spp.) and ponyfishes (Leiognathus spp.) (FAO, 2013; Klangnurak & True, 2022). Over exploitation of fish stock within Thai waters is an ongoing issue. Due to overfishing, across a 10 year period Thailand saw a 39% reduction in fishery production (Wongrak et al., 2021). Although this change was partially stemmed by reductions in illegal fishing since 2015; evidence of over exploitation is still present, such as with the decreasing trends in Indo-Pacific mackerel length from   18cm   to    15cm   (FAO,   2013).   Previous   stomach   content   analysis   of O.brevirostris subpopulations in the Eastern Gulf Coast of Thailand suggested their diet mainly piscivorous with a preference of ponyfishes, mackerel, and scad (Jackson-Ricketts, 2016). Although this is suggestive of resource overlap, few studies have quantified the relationship within this region and thus further research is warranted.

Part 2 - Description of the Proposed Research

Rationale

Despite O.brevirostris being legally protected (Partnership, 2010); their population is still on the decline having moved from “Vulnerable” to “Endangered” on the IUCN Red List (Minton et al., 2017) suggesting current conservation approaches are insufficient at stemming it. The death of only 4.2 individuals a year across a period of 60 years (3 generations) would result in a further 50% decline in population size (Moore, 2015). With sub populations generally ranging from 10 – 100 in size; mass mortality events as a result of fishery interactions such as the aforementioned death of fourteen O.brevirostris in the  Trat  Province  of Thailand  (Hines  et  al.,  2015) far  exceed this  protectory. Furthermore, decrease in the biomass of fish stocks due to overfishing not only affects commercial fisheries   but  sub-optimal   prey   could  affect O.brevirostris growth  and   reproduction  further exacerbating their decline  (Minton  et al., 2011). Therefore,  it  is  essential  a  proactive  data  led conservation stance is taken to avoid this species extinction.

Data scarcity has thus far limited ecosystem-based fishery management approaches (Cheng et al., 2022). In other marine ecosystems; increases in marine mammal populations when not monitored alongside the continuing growth in commercial fisheries has led to further damage of fish stocks (Jusufovski et al., 2019). In order for effective conservation to take place; it is essential to have a better understanding of the overlap between fisheries and the O.brevirostris. With projects currently underway  by the  UN  Environment  Programme  that  look to  integrate conservation  in to fishery management through the creation of “Regional System of Fishers Refugia in the South China Sea and the Gulf of Thailand” (Siriraksophon, 2022), now is the time to ensure sufficient data is available to guide these projects.

Research objectives and outcomes

1.   To analyse the O.brevirostris prey consumption rate and prey composition within the Trat

Province, Thailand, by stomach content analysis of stranded animals

2.   To establish resource overlap by comparison of the O.brevirostris prey consumption rate

to fisheries catch rate

3.   To compare the spatial overlap between O.brevirostris habitat (through use of electronic

tags) to local fishing grounds, by tracking fishing vessels in the area

Methodology and approach

The Gulf of Thailand is a continental shelf bordering Thailand, Cambodia, Malaysia & Vietnam (between latitudes 5。00’ and 13。30’ N and longitudes 99。00’ and 106。00’ E) (Jackson-Ricketts, 2016). This research will focus on previously sighted (Ponnampalam et al., 2013) O.brevirostris populations in the Trat Province, an area within Thailand’s EEZ (see figure 1).

Figure 1: Map highlighting the Trat Province (in yellow) within the Gulf of Thailand. Adapted from

Hines et al. (2015).

To understand the composition of the O.brevirostris prey, the stomach content of stranded animals will be collected and analysed. Stranded dolphins will be located through use of the Phuket Marine Biological Centres  (PMBC) stranding  network. Any  deceased dolphins will  have their stomach removed and frozen for  laboratory analysis. Once thawed stomach contents will  be  sieved to separate hard and soft tissues. Fish bone & otoliths will be dried; whilst cephalopod beaks will be stored in ethanol. Identification will be done using published guides (Giménez et al., 2017).  The relative  importance  of  each  prey  type  to  the  Irrawaddy  will  be  quantified  using  equations  in established in Giménez et al. (2017) based on proportion of prey and prey type against the total number of prey found within the stomach. Prey weight will be reconstructed by standard regressions. Overall annual consumption rate will be calculated using methods established by Santoset al. (2014) where O.brevirostris population size will be estimated based on an averages of the area found in Hines et al. (2015).

To established the geographical range of the O.brevirostris bio-tagging will be administered by modified methods from Wells et al. (2021). In brief, personnel and boat resources from PMBC will assist in the tagging process. Capture sites will be within shallow depths (less that 3m). Once spotted, O.brevirostris would be captured by encirclement using seine nets before being transferred to boats. Staff will be nominated to monitor in case of any entanglements; and to ensure they are kept wet with sponges once on the boats. A veterinarian will be present to continuously monitor captured animals. SPLASH10 tags will be attached to dorsal fins by a single pin to transmit location data  via  satellite.  As  fishing   is  carried  out   both  during  the  day  and  night  in  this  region (Noranarttragoon, 2014) it is essential a full day is covered so tags will be programmed to transmit Global Positioning System (GPS) data every 15 minutes (Hayset al., 2021). Data would be collected using  the  Argos  Data  Collection  and  Location  System.   For  sample  size  to  be  a  significant representative of the population in this region  10 individuals will be tagged in each deployment biannually.

By  Thai  law  commercial  fishing  vessels   must   register  with  Department  of   Fisheries  (DOF) (Kulanujaree et al., 2020). To establish the geographical range and movements of fishing vessels in the  Trat  region  geographical  data  will  be  collected  from  the  Marine  Fisheries  Research  and Development Division within the DOF (Tepparos et al., 2022).  For Vessels ≥30 GT that must be fitted with a vessel monitoring system (VMS), this will take the form. of hourly timecoded coordinates transmitted to the DOF (Tepparos et al., 2022). Vessels between 10 to <30 GT it will be in the form. of log books; and for artisanal and commercial fishing vessels less that 10 GT data it will be from DOF during field surveys (Tepparosetal., 2022). Catch composition would be estimated via monthly on-board observations across the aforementioned fishing vessel categories; as well as via observations of landing sites (represented by fish weight in kg) (Noegroho et al., 2020). In order to establish relationships in annual catch rate with the O.brevirostris, Catch per unit effort (CPUE) will be calculated using equations as per Noegroho et al. (2020).

To analyse resource overlap, fisheries annual catch rate will be compared to O.brevirostris annual consumption rate via a t-test. Irrawaddy prey composition and fisheries catch composition of each species will be compared via ANOVA. Post Hoc tests will be run to show where significance overlaps lie between fish species. To analysis spatial overlap, O.brevirostris and Fisheries positioning data will be mapped using ArcMap10.0 with the R-ArcGIS Bridge for statistical analysis using R.

Risks and mitigation strategies

If no dolphins are stranded during the research period, data from previously analysed O.brevirostris stomachs will be used as logged within the PMBCs stranding network. However; data from the research period will be preferential as it is based on current predator-prey relationships.

SPLASH10 tags have been known to fail. The largest cause of transmission loss is from battery failure (seen with 87% of cases) with a steep reduction in transmission rates following 200 days (Hays et al., 2021). As O.brevirostris have shown seasonal variance in movements (Tubbs et al.,

2020); tags will be deployed biannually to ensure data is collected across all season over a year helping abate this risk.

Programme of research

•    January 2023 - April 2023: Application for permits from the Foreign Researcher Management

Section of the National Research Council of Thailand

•   April 2023 - April 2024: Data collected from both dolphins and fisheries. SPLASH10 tags will

be fitted in April 2023 and October 2023

•    May 2024 - December 2024: Data analysed and paper prepared

Management of the project and resources

This project will be supported by Department of Marine and Coastal Resources with specific support from the PMBC. Research Assistant from the communities in the Trat province will be essential to ensuring the success of the project. They will be trained to take catch measurements and assist in tagging procedures.

Impact Summary

This paper, published in Marine Mammal Science, will serve to give a better understanding of how O.brevirostris and fisheries interact. If a significant overlap is found both in territory and prey-type this  paper will  aid  ecosystem-based fisheries  management.  This will  not  only  benefit  fisheries through improving catch yields and curbing decline in fish biomass; it will also lead conservation strategies that will prevent further decline of O.brevirostris populations. A greater understanding of the spatial distribution of O.brevirostris in this territory could be used in conservation of other marine sub-populations in similar habitats. Furthermore, improvements could indirectly lead to economic growth via dolphin-watching tourism.