Supplementary Information: Data sources and additional analyses used in section 4 (Consequences of prohibiting dFAD use)
For the analyses of spatio-temporal patterns of tropical tuna, publicly accessible purse seine (PS) and pole-and-line (BB) catch-effort data were downloaded from each of the four tuna regional fishery management organizations (tRFMO). Specifically, the following data sources were used:
• Atlantic Ocean (AO): Task 2 catch data raised to total landings on 5∘×5∘ were downloaded from the ICCAT website (https://www.iccat.int/Data/Catdis/cdis5020_all.7z, Accessed 2022-04-08)
• Indian Ocean (IO): Catch data for all fishing gears and fleets, generally raised to total
landings, were downloaded from the IOTC website
(https://www.iotc.org/sites/default/files/documents/2022/02/IOTC-DATASETS-2022-02- 23-CEAllGears_1950-2020.zip, Accessed 2022-04-08). Spatial level of aggregation for purse seine data was 1∘×1∘ , but this was further aggregated to the 5∘×5∘ level for all analyses
• Eastern Pacific Ocean (EPO): Purse seine catches by set type and pole-and-line catches, both at a 1∘×1∘ spatial level of aggregation, were downloaded from the IATTC website (https://www.iattc.org/PublicDomainData/PublicPSTuna.zip and https://www.iattc.org/PublicDomainData/PublicLPTuna.zip, respectively; Accessed on 2022-04-09). The catches in these data were then raised to the total catch reported in
non-spatial catch data by year, gear and species
(https://www.iattc.org/PublicDomainData/CatchByFlagGear.zip, Accessed on 2022-04- 09)
• Western-Central Pacific Ocean (WCPO): Purse seine and pole-and-line catch data aggregated at the 5∘×5∘ -year-month spatio-temporal scale were provided by Peter Williams ([email protected]) of the Pacific Community on 2022-11-07. The supplied data differ from publicly available data that can be downloaded from the WCPFC website (https://www.wcpfc.int/wcpfc-public-domain-aggregated-catcheffort-data-download- page ) in that non-spatial, declared annual catch estimates for Indonesia and the Philippines have been spatialized by evenly distributing catches over the EEZ of the corresponding country to avoid underestimating purse seine and pole-and-line catches for these countries for which spatial data is poor or non-existent, but non-spatial total catch data is reasonably reliable.
For PS catch declarations in the AO, IO and EPO, no separation was made in publicly accessible data among the different types of floating objects. The data could, therefore, contain small amounts of catch on anchored FADs (aFADs) and catch on natural floating objects (e.g., logs).
AFADs are generally rare in these zones and recent analyses indicate that natural floating objects represent <10% of all floating objects in the AO and the IO (Dupaix et al., 2021; Maufroy
et al., 2017). As such, we consider that the vast majority of floating object catches in these areas to be on dFADs.
The PS floating object catch declarations from the WCPO were divided into dFAD, aFAD and log categories. For the purpose of examining the impact of a dFAD ban, WCPO dFAD and log catches were grouped together into a single “dFAD” catch value as a ban on dFAD tracking buoys would likely also significantly impact log sets as opportunistically-encountered logs are regularly equipped with tracking buoys.
For analyzing the catch size (by large size category) and species composition of different types of PS sets, fine-scale logbook data from the Spanish and French fleets in the AO and IO were used.
Catch species composition was corrected using the T3 algorithm (Pianet et al., 2000).
For analyzing detailed size-frequency patterns of IO catch of yellowfin tuna on PS dFAD and FSC sets, public size-frequency data was downloaded from the IOTC website (https://iotc.org/sites/default/files/documents/2022/12/IOTC-DATASETS-2022-12-19-SF-YFT- 1952-2021.zip, Accessed 2023-03-29). These data were limited to EU and Seychelles PS fleets with high-quality size-frequency data and aggregated across spatial and temporal strata for the period 2010-2020, weighting data by the total declared catch over the same period in each
5∘×5∘ grid cell and by fish school type.
Summary of the data
The PS dFAD fishery harvested worldwide on average 1,501,952 mt of tropical tunas per year over the period 2010-2020, the majority of which is destined for the canned tuna market. Of this catch, 71.2% is composed of skipjack tuna (Table S.1) compared to 62.9% in non-dFADs PS sets (Table S.2). The remainder of the tropical tuna catch in dFADs sets is composed primarily of juvenile yellowfin tuna and juvenile bigeye tuna (Table S.3).
Table S.1: Catch in PS-dFAD sets by large fishing zone and the percentage of that catch that is each of the three major tuna species. Indicated catch is in thousands of mt and corresponds to the total for the period 2010-2020. Percentages sum to 100% for each row of the table.
Ocean Catch trop. tunas (1000 mt) % bigeye % skipjack % yellowfin
Atlantic 2,492 9.2 74.3 16.5
Indian 2,953 7.2 60.9 31.9
Pacific, Eastern 3,373 20.0 64.3 15.7
Pacific, Western-Central 7,703 7.5 77.2 15.3
Worldwide 16,521 10.2 71.2 18.6
Table S.2: Catch in non-dFAD PS sets by large fishing zone and the percentage of that catch that is each of the three major tuna species. Catch is predominantly on free-swimming schools, though it includes relatively small amounts of catch from unclassified sets and, for the Western- Central Pacific, non-negligible catch on aFADs. Indicated catch is in thousands of mt and corresponds to the total for the period 2010-2020. Percentages sum to 100% for each row of the table.
Ocean Catch trop. tunas (1000 mt) % bigeye % skipjack % yellowfin
Atlantic 830 6.2 27.3 66.5
Indian 691 9.2 22.8 67.9
Pacific, Eastern 2,960 0.4 33.3 66.3
Pacific, Western-Central 12,164 1.5 74.9 23.7
Worldwide 14,644 1.8 62.9 35.2
Table S.3: Percentages by weight of yellowfin tuna and bigeye tuna individuals that are <10 kg and >10 kg (generally taken as the break between juvenile and mature individuals in these two species) among individuals caught on various fish school types in the purse seine fisheries of the Atlantic and Indian Oceans over the period 2010-2020. Data are derived from T3-corrected logbook data from the Spanish and French fleets.
School type % YFT <10 kg % YFT >10 kg
% BET <10
kg % BET >10 kg
dFAD 68.1 31.9 84.4 15.6
FSC 3.9 96.1 6.5 93.5
Unclassified 20.2 79.8 32.7 67.3
Other analyses
Consequences of shift from dFAD to FSC fishing on yellowfin tuna purse seine catch.
If a ban on dFAD fishing leads to increased fishing on FSC, then this will lead to a shift in the size composition of yellowfin tuna catch by PS. Whereas dFAD catch of yellowfin tunas is dominated by individuals that are around 50 cm fork length (~1-2 years old), FSC catch is dominated (by
weight) by individuals that are approximately 130 cm fork length (~3-4 years old) (Chassot et al., 2013, Figure S.1). Assuming yellowfin tuna populations are not recruitment limited (Fonteneau et al., 2000), whether or not this increase in size would lead to an increase in harvestable biomass of yellowfin tuna will depend on the mortality rate between the age at which the yellowfin would have been caught under dFADs and the age at which they are caught in FSC. If, for simplicity, we assume that natural mortality rate estimates are representative of the total mortality rate over the yellowfin tuna age classes with which we are concerned (i.e., we ignore the mortality of juvenile yellowfin tuna in other fisheries, such as pole-and-line fishing), then we can approximately estimate the change in harvestable biomass by multiplying the fractional increase in fish weight as a fish grows from 50 cm to 130 cm by the probability that an individual does not die due to natural mortality during the time between its typical age at 50 cm to its typical age at 130 cm (if, on the other hand, fishing mortality is non-negligible for these size classes even in the absence of dFAD fishing, then this will have a tendency to raise the total mortality rate and, therefore, reduce or eliminate any increase in harvestable biomass).
Figure S.1: Relative length-frequency of purse seine catch by fish school type for the Indian Ocean, 2010-2020. Frequency is in relative numbers of individuals.
To carry out this calculation, we used the yellowfin tuna age-length relationship in Dortel et al.
(2015) (specifically the mean parameter estimates for the Gamma distribution model, Table 4), the length-weight relationship in (Zhu et al., 2008), and the natural mortality rates estimated by (Hampton, 2000). The mean weight as a function of length over the three geographical areas
examined by Zhu et al. (2010) (i.e., the AO, IO and EPO) was used as our overall weight prediction. Hampton (2000) assessed that mortality rates varied considerably as a function of size, with rates ranging from 0.44 yr-1 for individuals of length 61-70 cm to 2.2 yr-1 for individuals of size 91-100 cm. We, therefore, varied the natural mortality rate between these two bounds to assess change in fishable biomass as a function of mortality.
Results indicate that, for the range of potential mean natural mortality rates possible for yellowfin tuna between 50 cm and 130 cm for length, both increases and decreases in harvestable biomass are possible (Figure S.2). Changes in harvestable biomass ranged from a low of 0.084 times the current biomass to a high of 6.292 times the current biomass, with harvestable biomass remaining unchanged at a natural mortality rate of 1.2 yr-1.
Figure S.2: Estimated relative change in yellowfin tuna harvestable biomass as a function of mean instantaneous mortality rate if purse seine fishing effort is shifted from dFAD-associated schools, consisting primarily of juvenile individuals, to free-swimming schools, consisting primarily of larger, mature individuals.
The calculation described above has a number of limitations, including the assumption of a single size for dFAD and FSC yellowfin tunas and the use of a single length-weight relationship.
These weaknesses were addressed in additional calculations by using the length-frequency data in Figure S.1 to develop a probability of transitioning from a given size at capture in a dFAD school to a size at capture in a FSC (i.e., the product of the two relative length-frequencies) and by developing results for all the yellowfin tuna length-weight relationships presented in Zhu et
al. (2010). For each length class, age was calculated by inverting the age-length relationship in Dortel et al. (2015), assigning all individuals longer than the L∞ to age 7, an age at which Dortel et al. (2015) would predict a size within 0.11% of L∞. Note that this choice of a relatively young maximum age compared to the maximum reported age for yellowfin tuna of up to 18 years (Pacicco et al., 2021) will tend to favor increased fishable biomass when fishing on free- swimming schools, though this choice is potentially justified by the fact that natural mortality rates for large, mature yellowfin are generally considered to be lower than those for juveniles and young adults.
The results of this more sophisticated calculation are presented in Figure S.3. Results are qualitatively similar to those from the simpler initial calculation (Figure S.2), with the mortality rate at which there is no change in fishable biomass ranging from 1.06 yr-1 to 1.17 yr-1. Though changes are modest, this more sophisticated calculation tends to decrease changes in fishable biomass when transitioning to free-swimming schools as free-swimming school sets do catch some juvenile individuals, lowering the effective mean age of individuals caught (Figure S.1;
though note that in terms of biomass the free-swimming school curve would be heavily skewed towards larger individuals).
Figure S.3: Estimated relative change in yellowfin tuna harvestable biomass as a function of mean instantaneous mortality rate if purse seine fishing effort is shifted from dFAD-associated schools to free-swimming schools based on the difference in size-frequency shown in Figure S.1.
The different colored curves show results for the different length-weight relationships in Zhu et al. (2010) (and the mean of those relationships shown in red), and the vertical dashed lines indicate the mortality rates at which fishable biomass would be the same for catch on dFAD
schools versus catch on free-swimming schools for the length-weight relationship of the corresponding color.
These more sophisticated calculations still have a number of deficiencies, including the simple inversion of the age-length relationship instead of using a length-age key and the assumption of constant (natural) mortality between all age transitions. Addressing these issues would likely require a stock assessment model integrating mortality rates for multiple fisheries and length- age keys. Nevertheless, these results are sufficiently robust to indicate that the effect on fishable biomass of a transition from fishing on dFADs to fishing on free-swimming schools is uncertain, with both positive and negative effects possible.
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