Research ArticleECOLOGY

Overfishing and habitat loss drive range contraction of iconic marine fishes to near extinction

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Science Advances  10 Feb 2021:
Vol. 7, no. 7, eabb6026
DOI: 10.1126/sciadv.abb6026
  • Fig. 1 Linking dynamic geography to abundance-occupancy.

    (A) Changes in abundance-occupancy with varying levels of fishing shown in different shades of red. Slope tangent to the line represents r, the population growth rate, which is synonymous with, and indeed a definition of, habitat quality. Increasing fishing pressure causes the occupancy curve to approach its asymptotic limit for a smaller given abundance compared to no fishing pressure. The maximum abundance of a given population shrinks under stronger fishing regimes, as shown with the point and the dashed line. (B) Curves derived from (A) showing changes in habitat quality (= r) with habitat availability with varying levels of fishing. When fishing pressure is high, the abundance-occupancy curve approaches its asymptotic limit (r = 0) at a lower given occupancy (A), resulting in a steeper decline in population growth rate and habitat quality for a given available habitat, resulting in a geographic range contraction.

  • Fig. 2 The historical presence, extinction, and uncertainty of the presence of sawfishes.

    (A) Global sawfish search effort with each color representing the different activities and the size of the point representing the number of activities in each nation, where the smallest point represents one activity and the largest point represents 14 activities. (B) The historical distribution of sawfish species richness across 90 nations. (C) The number of sawfish species extinct in each nation. (D) The number of sawfish species with Presence Uncertain status; no color means the presence status is known. For (B) to (D), statuses are colored in the exclusive economic zone (EEZ) of each nation’s coastal waters and where greater species richness is denoted by the warmer colors.

  • Fig. 3 Marginal effects of ecological carrying capacity, fishing pressure, and management capacity on extinction risk in sawfishes.

    Partial dependence plots calculated for each predictor when all other indices are averaged. The solid black curve is the median of 1000 bootstrapped samples and the shaded ribbon shows the minimum and maximum fitted response for (A to E) ecological carrying capacity [(A) shelf area (km2; 25.0%), (B) mangrove area (km2; 14.8%), (C) estuarine discharge rate (m3 s−1; 3.06%), (D) marine primary productivity (mg m−3; 2.10%), and (E) sea surface temperature (°C; 1.52%)], (F to J) indices of fishing pressure [(F) gear-specific landings (metric tons; 14.5%), (G) marine protein consumption (metric tons; 9.38%), (H) total chondrichthyan landings (4.86%), (I) coastal human population (4.82%), and (J) fishing effort (2.22%)], (K to M) management capacity [(K) World Governance Index (WGI; 7.18%), (L) Gross Domestic Product (GDP; 5.66%), and (M) Human Development Index (HDI; 3.67%)], and (N to R) species identity [(N) Smalltooth Sawfish, P. pectinata (0.54%); (O) Green Sawfish, P. zijsron (0.42%); (P) Largetooth Sawfish, P. pristis (0.21%); (Q) Narrow Sawfish A. cuspidata (0%); and (R) Dwarf Sawfish, P. clavata (0%)]. The response is shown with standardized values of predictor variables (calculated by subtracting the mean and dividing by the SD) for presentation purposes, whereas the analysis was run with unscaled values.

  • Fig. 4 Dynamic geography of sawfish populations.

    The effects of increasing fishing pressure (gear-specific landings), habitat quality (mangrove area), and habitat availability (shelf area) on occupancy in sawfishes. (A) Logistic regression where the thin curves show draws from the posterior distribution and the thick colored curves are the mean posterior estimates. Curves are colored and predicted by levels of fishing pressure (where mangrove area is held at its mean): zero fishing shown in the lightest orange, low fishing in orange, moderate fishing in red, high fishing in dark red, and maximum fishing in darkest red. The thick gray line shows the intersection where 5% occupancy occurs. Light gray rugs show the data. (B) Posterior distributions of the coefficient estimates from the logistic regression for shelf area (blue), mangrove area (blue), and fishing pressure (i.e., gear-specific landings; red), where the majority of the posterior is darker. Shelf area had a strong positive effect on the occupancy of sawfishes [mean estimate = 4.08, 95% credible interval (CI) = 1.52 to 8.05; 100% of the posterior > 0], whereas mangrove area had a small positive effect (0.48, 95% CI = −0.99 to 2.54; 72.1% of the posterior distribution > 0), and fishing pressure had a strong negative effect on the occupancy of sawfishes (−1.17, 95% CI = −3.05 to 0.03; 97.2% of the posterior distribution < 0). (C) Estimated habitat required to have 5% occupancy drawn from the posterior distribution through different levels of fishing. Violin plots and points show spread of the predicted draws and thick lines show the median value. Points have been jittered for ease of interpretation.

  • Fig. 5 Sawfish extinction risk and national conservation potential.

    (A) Predicted probability of extinction from 1000 bootstrapped BRTs combined with current nations of occurrence represented in the EEZ. (B and C) Changes in predicted probability of extinction (current risk in dark colored points, predicted risk in transparent points) for Presence Uncertain nations where (B) fishing mortality (except coastal human population) is zero and (C) mangrove area is doubled. Dark blue, nations where sawfishes are present or have the lowest probability of extinction (<0.2); light blue, low probability of extinction (0.2 to 0.4); lightest blue, moderate probability of extinction (0.4 to 0.6); red, high probability of extinction (0.6 to 0.8); dark red, extremely high probability of extinction (0.8 to 1.0) or are already recorded as locally extinct.

  • Table 1 Variables considered in the BRT model.

    Only variables included in the model are shown.

    VariableDescriptionSupport
    Indirect fishing pressures
      ln coastal populationNumber of people living in urban and rural areas
    within 100 km of the nation’s coastline as of 2011
    Large coastal populations may drive unsustainable
    fishing practices (30)
      ln marine protein consumptionMarine fish food supply, g capita−1 day−1The reliance on marine fish products for dietary
    protein and economic stability (42)
    Direct fishing pressures
      ln chondrichthyan landingsTotal metric tons summed for all Aquatic Sciences
    and Fisheries Information System species of
    sharks, rays, and chimaeras. Total of 146
    species-specific and 23 aggregate not elsewhere
    indicated categories
    Shark, ray, and chimaera products are of high
    economic value (59); thus, sawfishes that are
    caught opportunistically would have high
    economic return (28)
      ln gear-specific marine fisheries landingsTotal metric tons summed for catches with
    bottom trawls, gillnets, otter trawls, shrimp trawls,
    small-scale gillnets, small-scale longlines,
    small-scale trammel nets, and trammel nets
    Sawfishes have high catchability in specific fishing
    gears (53)
      ln fishing effortFishing effort (kW) for artisanal and subsistence
    sectors from the EEZ of each nation
    Sawfishes have high catchability in small-scale,
    in-shore fisheries (53)
    Management capacity
      World Governance IndexOn the basis of the control of corruption
    government effectiveness, political stability, rule
    of law, regulatory quality, and voice and
    accountability
    Governance is required for effective and
    sustainable fisheries management (60)
      ln Gross Domestic ProductMeasured in million USDNations with high income and high development
    status have better fisheries management than
    low-income countries (60, 61)
      Human Development IndexMeasures life expectancy, education, and Gross
    National Income
    Nations with high income and high development
    status have better fisheries management than
    low-income countries (60, 61)
    Ecological carrying capacity
      ln continental shelf areaMeasured as the area found within the distribution
    maps for each species clipped to the maximum
    depth of each reported species (km2)
    Larger habitats can potentially support larger
    sawfish populations (62)
      ln marine primary productivityChlorophyll a per nation, mg m−3Marine primary productivity linked to diversifying
    food webs in marine habitats by supporting
    higher trophic guild individuals (63)
      ln estuarine discharge rateMean freshwater input, m3 s−1Sawfishes are highly associated with river and
    estuarine systems (25, 64)
      ln mangrove areaMean mangrove area (km2)Sawfishes are highly associated with mangroves
    (53)
      Sea surface temperatureMonthly means from 1981 to 2018 in °CSawfishes are associated with tropical waters with
    a lower lethal temperature between 8° and 12°C
    (64)

Supplementary Materials

  • Supplementary Materials

    Overfishing and habitat loss drive range contraction of iconic marine fishes to near extinction

    Helen F. Yan, Peter M. Kyne, Rima W. Jabado, Ruth H. Leeney, Lindsay N.K. Davidson, Danielle H. Derrick, Brittany Finucci, Robert P. Freckleton, Sonja V. Fordham, Nicholas K. Dulvy

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