Research ArticleECOLOGY

Shark baselines and the conservation role of remote coral reef ecosystems

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Science Advances  07 Mar 2018:
Vol. 4, no. 3, eaaq0333
DOI: 10.1126/sciadv.aaq0333
  • Fig. 1 The BIOT (green transparent polygon in the inset map) and the Chagos archipelago.

    Dots are locations of illegal fishing intercepted by the BIOT patrolling vessel, the Pacific Marlin.

  • Fig. 2 Abundance indices and estimated catches of silvertip and gray reef sharks in the Chagos archipelago.

    Time series of the indices of abundance obtained from scuba diving surveys for silvertip (A) and gray reef sharks (B). Trend lines in (A) and (B) were predicted for illustrative purposes by fitting a weighted quadratic regression (Ot = year + year2 + ϵ). Shown are trends both within the USSR and guess scenario of community structure for the 1970s when species-specific indices were unavailable (see the text). (C) Estimated total catch of species from longline and IUU fisheries in the BIOT. Dashed lines are posterior means of estimated catches during the period 1975 to 1995 under a scenario of censored catch records (that is, in this period, our catch reconstruction for the two species was a gross underestimation; see Results). The posterior means of estimated catches referred to the best models for each species respectively.

  • Fig. 3 Population trends of gray reef (green) and silvertip sharks (blue) as estimated by the SPMs.

    Trajectories are drawn only for periods when Ot and Ct data exist and overlap. Lines are mean population estimates and transparent polygons are 80% credible intervals (CI). Carrying capacity is represented by the initial flat line and CI before 1948 (that is, we estimated that population baselines occurred sometime earlier than 1948 for both species).

  • Table 1 Timeline of exploitation, management, and conservation events with a likely effect on the focal reef shark species around the Chagos archipelago.
    DateEventReference
    1927Beginning of industrial exploitation of Mauritian offshore banks including the Chagos archipelago.(40)
    1948First handline scientific surveys are being carried out in several tropical banks
    of the WIO including the Chagos archipelago.
    (36)
    1949Mauritian bank fishery expands to several banks in the central WIO.(39)
    1952Longlining was first introduced in the Eastern Indian Ocean and soon spread across the entire ocean.(32)
    1965The Chagos archipelago is detached from the British colony of Mauritius for
    joint UK/U.S. defense purposes.
    (61)
    1967Indian Ocean Fishery Commission established.(62)
    1971Construction of a military base started on Diego Garcia.(24)
    1972The UK government relocated the last few Chagossians from the Chagos archipelago.(61)
    1973The Taiwanese fleet expanded to target the whole WIO.(30)
    1975First scuba diving surveys are being carried out around the Chagos archipelago.(23)
    1977Mauritian bank fishery data became available from the Chagos archipelago.(39)
    1982The Indo-Pacific Tuna Development and Management Programme (IPTP) was
    established to manage the rapidly growing industry in the region.
    (62)
    Mid 1980sIncreasing demand and markets for shark result in growth in shark fishing in the region.(23)
    1989Total tuna catch in the Indian Ocean exceeds that in the Atlantic Ocean for the first
    time and has never fallen below it (Pacific tuna catch still exceeds both).
    (30)
    1991Establishment of a 200-mile FCMZ.http://mrag.co.uk/experience/management-british-indian-ocean-territory-fisheries-regime
    1996FCO begins to report illegal fishing vessels incurring in the BIOT.(28)
    1997The IPTP is replaced by the IOTC whose remit no longer extends into the Western Pacific.(62)
    2000Start of the fisheries observer program in BIOT.(58)
    2005–2009Somali piracy results in fishing effort being displaced away from the Somali coast to
    other areas of the Indian Ocean and an overall 25% reduction in fleet capacity
    (63)
    9 Nov 2009Public consultation on establishing a large marine reserve in BIOT opened.(9)
    2009Maldives ban reef shark fishery within their atolls.(64)
    2010Maldives completely ban shark fishery within their jurisdictional waters.(64)
    2010The Maldives extend a national ban on shark hunting, banning shark fishing in all its
    waters and all shark product exports. This decision was based on evidence that sharks
    are more valuable as a tourist attraction than as a fishery resource.
    (64)
    10 Apr 2010A no-take BIOT marine reserve is established around the BIOT territorial waters, and all
    fishing activities are banned within the 640,000-km2 boundary.
    (9, 21)
  • Table 2 Summary statistics for the posterior estimates of population abundance (PopEs), carrying capacity (K), maximum population growth rate (r), coefficient q, and catch factor (ψ) under different modeling scenarios.

    BS, baseline; SS, seventies scenario. The SDs of the posterior distributions are shown in parentheses. Model fit is indicated by its relative WAIC.

    SpeciesBSSSPopEsKrqψWAIC
    1Silvertip shark (STS)MacroecoGuess31,693 (18,513)455,337 (31,780)0.045 (0.001)4.0 × 10−6 (1.4 × 10−6)0.0155 (0.0031)2330.21
    2USSR28,623 (17,057)456,142 (30,553)0.045 (0.001)4.4 × 10−6 (1.6 × 10−6)0.0152 (0.0029)2444.74
    3WheelerGuess20,122 (13,336)216,709 (15,059)0.045 (0.001)6.2 × 10−6 (2.3 × 10−6)0.0344 (0.0079)2632.70
    4USSR18,595 (12,381)217,439 (14,784)0.045 (0.001)6.6 × 10−6 (2.5 × 10−6)0.0343 (0.0081)2341.33
    5Gray reef shark (GRS)MacroecoGuess586,114 (154,059)722,930 (42,733)0.125 (0.005)2.0 × 10−6 (3.2 × 10−6)0.0058 (0.0054)2042.31
    6USSR571,310 (124,915)727,435 (43,353)0.125 (0.005)1.6 × 10−6 (1.6 × 10−6)0.0075 (0.006)1788.60
    7WheelerGuess439,654 (355,158)2,330,047 (164,414)0.124 (0.005)4.4 × 10−6 (3.1 × 10−6)0.0038 (0.0026)2708.90
    8USSR645,275 (344,663)2,325,847 (162,766)0.123 (0.005)2.4 × 10−6 (1.6 × 10−6)0.0045 (0.003)2359.70

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/3/eaaq0333/DC1

    Supplementary Materials

    fig. S1. Standardized CPUEs of elasmobranchs detected in the USSR surveys.

    fig. S2. Summary of shark catches from seizure reports.

    fig. S3. Parameter estimates of the models standardizing the USSR surveys’ CPUE.

    fig. S4. Meta-analysis of baseline shark densities.

    fig. S5. Handline survey’s CPUE.

    fig. S6. Baseline community composition as estimated from ecological theory.

    fig. S7. Posterior means and credible intervals of shark population abundances.

    fig. S8. Catches of vessels intercepted by the BIOT patrolling vessel.

    fig. S9. Geographic distribution of the USSR longline sets.

    fig. S10. Pictures of seized shark catches, a BIOT patrolling vessel, and a typical illegal fishing boat entering the BIOT.

    table S1. Input life histories for Carcharhinus amblyrhynchos.

    table S2. Input life histories for Carcharhinus albimarginatus.

    table S3. Summary statistics of the SPMs.

    Model output files

    Reference (65)

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Materials
    • fig. S1. Standardized CPUEs of elasmobranchs detected in the USSR surveys.
    • fig. S2. Summary of shark catches from seizure reports.
    • fig. S3. Parameter estimates of the models standardizing the USSR surveys’ CPUE.
    • fig. S4. Meta-analysis of baseline shark densities.
    • fig. S5. Handline survey’s CPUE.
    • fig. S6. Baseline community composition as estimated from ecological theory.
    • fig. S7. Posterior means and credible intervals of shark population abundances.
    • fig. S8. Catches of vessels intercepted by the BIOT patrolling vessel.
    • fig. S9. Geographic distribution of the USSR longline sets.
    • fig. S10. Pictures of seized shark catches, a BIOT patrolling vessel, and a typical illegal fishing boat entering the BIOT.
    • table S1. Input life histories for Carcharhinus amblyrhynchos.
    • table S2. Input life histories for Carcharhinus albimarginatus.
    • table S3. Summary statistics of the SPMs.
    • Reference (65)

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    Other Supplementary Material for this manuscript includes the following:

    Files in this Data Supplement:

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