Fisheries and Resources Monitoring System

Longtail tuna - Indian Ocean
Marine Resource  Fact Sheet
Stock status report 2021
Longtail tuna - Indian Ocean
Fact Sheet Citation  
Longtail tuna Indian Ocean
Owned byIndian Ocean Tuna Commission (IOTC) – More
Related observationsLocate in inventorydisplay tree map
FAO Names: en - Longtail tuna, fr - Thon mignon, es - Atún tongol

Fishery Indicators
Production: Catch
Geographic extent of Longtail tuna - Indian Ocean
Main Descriptors
Considered a single stock: Yes        Management unit: Yes
Reference year: 2019
Biological State and Trend
State & Trend Descriptors
Exploitation rateSubject to overfishing (F2019/FMSY (80% CI) = 1.52 (0.751 – 2.87))High fishing mortality
Abundance levelOverfished (B2019/BMSY (80% CI) = 0.69 (0.45 – 1.21))Low abundance
Habitat and Biology
Climatic zone: Temperate; Tropical.   Horizontal distribution: Oceanic.   Vertical distribution: Pelagic.  

Geographical Distribution
Jurisdictional distribution: Highly migratory

Water Area Overview
Geo References
Resource Structure
Considered a single stock: Yes

No information is available on the stock structure of longtail tuna in the Indian Ocean. The IOTC coordinated Stock Structure Project, which commenced in early-2015, aims to supplement gaps in the existing knowledge on biological data, and in particular provide an insight on whether neritic tuna and tuna like species should be considered as a single Indian Ocean stock.
Fishery Indicators
TypeMeasureValueUnitTime period
ProductionCatch 3564tonnesAverage 1950 - 1959
Catch 7230tonnesAverage 1960 - 1969
Catch 12681tonnesAverage 1970 - 1979
Catch 35901tonnesAverage 1980 - 1989
Catch 58852tonnesAverage 1990 - 1999
Catch 96029tonnesAverage 2000 - 2009
Catch 147223tonnesAverage 2010 - 2019
Catch 171633tonnes2011
Catch 176724tonnes2012
Catch 158193tonnes2013
Catch 148661tonnes2014
Catch 140008tonnes2015
Catch 141352tonnes2016
Catch 144970tonnes2017
Catch 136154tonnes2018
Catch 112914tonnes2019
Catch 132529tonnes2020

No new assessment was conducted for longtail tuna in 2021 and so the results are based on the assessment carried out in 2020 using the Optimised Catch-Only Method (OCOM). Analysis using the OCOM indicates that the stock is being exploited at a rate that exceeded FMSY in recent years and that the stock appears to be below BMSY and above FMSY (76% of plausible models runs) (Fig. 2). Catches were above MSY between 2010 and 2018 but steadily declined from 2012 to were less than 113,000 t in 2019, below the estimated MSY (Fig. 1). The F2018/FMSY ratio is slightly higher than previous estimates. The estimate of the B2018 /BMSY ratio (0.69) was lower than in previous years, reflecting declining abundance. An assessment using a biomass dynamic model incorporating gillnet CPUE indices was also undertaken in 2020 and results were consistent with OCOM in terms of status. Therefore, based on the weight-of-evidence currently available, the stock is considered to be both overfished and subject to overfishing (Table 1; Fig. 2).
Assessment Model
Type:  Biomass-aggregated
Optimised Catch Only Method (OCOM)
Scientific Advice

The catch in 2019 was below the estimated MSY but the exploitation rate has been increasing over the last few years, as a result of the declining abundance. Despite the substantial uncertainties, this suggests that the stock is very close to being fished at MSY levels and that higher catches may not be sustained. A precautionary approach to management is recommended.
Management unit: Yes
Source of information
IOTC–SC23 2020. Report of the 23rd Session of the IOTC Scientific Committee. Seychelles, 7 – 11 December 2020. IOTC–2020–SC23–R[E]: 211pp.  Click to open
All references to figures, tables and bibliography in the text are found within the source of information.
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