Weight and length measurements for 37 Mekong River fish species of the ‘Dai’ fishery

The Lower Mekong Basin supports some of the world’s largest and most diverse inland fisheries by supplying nutrition to over 60 million people and an estimated 11 billion USD of economic activity each year. Despite the nutritional and economic importance of the fishery, there are limited data available on which fishery officers and managers can build fishery stock management plans. Traditional fishery stock management relies on models of individual and population growth that in turn rely on robust individual-based data sets, preferably collected over long time scales. One such fishery for which there are robust data sets are for the “dai,” or bagnet, fishery located on the Tonle Sap River that connects the Mekong River mainstream with the Tonle Sap Lake. However, datasets from the dai fishery are not publicly available, limiting their utility for understanding how catches in this fishery have changed over time. Several papers have utilized the data from the dai fishery, but none have made the data publicly available. By partnering with commercial fishers who participate in the dai fishery and in partnership with the Cambodian Department of Fisheries and the Inland Fisheries Research and Development Institute we collected weight and length data bi-monthly for fish captured at two dai operations starting in November 2020 and continuing to February 2021, totaling 32,274 measurements of length and weight for 37 species. Here we report weight and length data for individuals captured during this timeframe as well as calculated weight-length relationships for total length and standard length for all 37 species for which there were ³30 measurements. Plots of all weight-length regressions are also included in the data package. Finally, we report equations for converting standard length measurements to total length estimates and vice versa.

Methods
Since at least the year 2000 the Cambodian Department of Fisheries has monitored the commercial fisheries catch at the ‘dai’ fishery on the Tonle Sap River. The dai fishery consists of a barrage of fishing extending laterally across the river at 15 locations (dai rows). Dai rows are numbered 2-16 and can include up to seven individual fishing platforms that deploy and raise large fishing nets into the water column. These nets measure approximately 25 meters in width, while the depth of the net is determined by the depth of the water column. At their peak, individual dai nets capture thousands of kilograms of fish per day. Monitoring of fish catch by the Department of Fisheries, now the Fisheries Administration (FiA), is conducted twice per month, corresponding with the high catch period in the seven days preceding the full moon, and the low catch period during the rest of the month during the period of time in which the Tonle Sap River flows from Tonle Sap Lake to the Mekong Mainstream at Phnom Penh. This fishing season (October – March) corresponds with the out-migration of fish from the Tonle Sap Lake, where they travel to feed and reproduce, before returning to the deeper waters of the Mekong River prior to the onset of the prolonged dry season (December – May). The protocol for collecting weight and length data is based on sub-sampling of the large haul from the dai trap net. Individuals of economically and culturally important species is identified to species, weighed, and measured for total length (anterior-most point to distal margin of compressed caudal fin) and standard length (anterior-most point to last vertebra). Fish weights are measured in the field using a digital balance with precision to 0.1 g and all lengths are recorded to the nearest 0.1 cm. Further details on the dai fishery can be found in (Halls et al. 2013, Ngor et al. 2018)

            Using these data we established length-weight relationship for all fish species following a two-parameter power function with a multiplicative error term:

Wi = αLiβ10∈i

where  and  are model parameters, Wi and Li are the weight and length (TL or SL), respectively, of individual i. For all species for which we collected >30 length measurements, we regressed length-weight relationships using the linearized form of the equation:

log10Wi = log10(α) + β log10Li + ∈i

For total length-weight relationships, 32,274 individual fish were measured belonging to 37 different species. All relationships were statistically significant (p < 0.001), and adjusted R2 values were generally high (> 0.9), indicating good model fits. Likewise, standard length-weight relationships for 32,205 individuals belonging to 37 species were all strongly significant (p <0.001) and generally showed good model fits. Model fits for particularly small species (< 5cm) are not as strong due to a lack of scale precision at low values.

Acknowledgments

The authors would like to acknowledge Chin Channa who collected the field data, Srey Keo Sopheak, the deputy head of Siem Reap Fisheries Administration Cantonment for help in coordinating field sampling in Chong Kneas, Siem Reap, Cambodia, the fishers helping with fish collection, and Chea Seila for coordination and logistics support.