Standardised size-frequency data for skipjack tuna

Each year, the IOTC Secretariat prepares reference size-frequency datasets for each IOTC species under its mandate to provide information on the size composition catches within the IOTC area of competence. These data are collected on land by enumerators during unloading and transshipment operations, and at sea by fishers and scientific observers from the IOTC Contracting Parties and Cooperating Non-Contracting Parties (CPCs), in accordance with IOTC data requirements (Resolution 15/02). The datasets are submitted annually, predominantly using IOTC Form 4SF, and curated by the Secretariat for dissemination ahead of the relevant IOTC Working Parties.

Global attributes

• Title: Standardised size-frequency data available in the IOTC database for Skipjack tuna
• Summary: Curated dataset providing detailed information on the length composition of tuna catch
• Context of development: Management of tuna and tuna-like species under IOTC mandate
• Data origin: Size samples collected on land or at sea by national institutions and the fishing industry, with some data having been previously extrapolated to represent the broader fishery
• Lineage: Formatting, filtering, and conversion of size data submitted to the Secretariat (Data processing)
• Keywords: Skipjack tuna, length, selectivity, size, tuna fisheries
• Type: Tabular files
• Format: CSV
• Size: 73,661 KB for melted dataset and 17,257 KB for pivot table dataset
• Structures:
  • 1,082,872 rows x 16 columns
  • 38,494 rows x 166 columns
• Taxonomic standard: ASFIS
• Geographic description: Fishing areas within the Indian Ocean
• Spatial extent: 20° to 120° longitude and -45° to 30° latitude for regular grids
• Temporal extent: 1982-2024
• Temporal extent start date: 1982-06-01
• Temporal extent end date: 2024-12-31
• Spatial resolution: Spatial resolution table
• Spatial standard: CWP for regular grid areas
• Temporal resolution: Temporal resolution table
• Date standard: ISO 8601
• Data quality: IOTC reporting quality scoring
• Update frequency: At least once per year
• Metadata production date: 10 October 2025
• Source: IOTC
• Contact: [email protected]
• Credit: IOTC Secretariat
• Associated rights: Public-domain data requiring source acknowledgement

Data processing

The document IOTC-2020-WPDCS16-16 describes the processing steps applied to the size-frequency data managed at the IOTC Secretariat:

1- Removal of all sizes reported with an interval larger than the maximum interval value (Size measurement features)

2- Conversion of all sizes into standard measurement type with reference to conversion methods available in IOTC-2025-WPTT27(AS)-DATA13

3- Distribution of the number of fish reported in intervals larger than the standard interval (Size measurement features) into equally spaced smaller intervals

4- Assignment of all standard lengths smaller than the minimum standard length class to the minimum standard length class (Size measurement features)

5- Removal of all standard lengths larger than the maximum standard length class

6- Removal of data reported for areas not in the Indian Ocean

Reporting quality scores

Spatial resolution

Temporal resolution

Fleets

Gears

School types

Measure types

Raisings