OpenUCT :: Browsing by Author "Branch, Trevor A"

Browsing by Author "Branch, Trevor A"

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Open Access
Abundance estimates for Antarctic minke whales from three completed circumpolar sets of surveys, 1978/79 to 2003/04
(2006) Branch, Trevor A
Abundance estimates are provided for Antarctic minke whales from the ship-based IDCR-SOWER surveys using the standard distance sampling methodology applied in the past in the Scientific Committee. Agreed methods of pooling strata and of estimating mean school size have changed since the most recent published assessment of these surveys by Branch and Butterworth (2001a). The IDCR-SOWER surveys are grouped into three completed circumpolar sets of cruises: 1978/79–1983/84 (CPI), 1985/86–1990/91 (CPII) and 1991/92–2003/04 (CPIII), which respectively covered 64.3%, 79.5% and 99.7% of the ice-free area south of 60°S. Circumpolar abundance estimates are obtained by summing individual surveys in CPI and CPII (each covered one IWC Management Area), and by combining CPIII surveys (some overlapped) using the ‘survey-once’ method—by selecting the single survey offering the best or most recent coverage. When calibrated closing and independent observer mode estimates were inverse-variance weighted, circumpolar abundance estimates were 645,000 (CV = 0.143), 786,000 (CV = 0.094) and 338,000 (CV = 0.079) for CPI, CPII and CPIII respectively. These estimates are negatively biased because some Antarctic minke whales are north of 60°S and inside the pack ice during the surveys, and because some whales on the trackline are missed. After simple extrapolation to account for differences in the latitudes surveyed during each circumpolar set and for the increasing proportions of ‘like minke’ sightings, the ratio of estimates from the three CPs is 0.97:1.00:0.39, echoing previous findings of appreciably lower CPIII estimates. CPIII estimates for individual IWC Management Areas are similarly low, ranging within 18–52% of CPII estimates for Areas I–V, although 159% of CPII for Area VI. Explanations for the appreciably lower abundance estimates include a higher proportion of minke whales within the pack ice and a greater proportion of whales missed on the trackline, but any such hypothesis needs to be reconciled with higher abundance estimates in CPIII than in CPII for blue, humpback, fin, sperm and killer whales based on the same surveys.
Open Access
Abundance of Antarctic blue whales south of 60°S from three complete circumpolar sets of surveys
(International Whaling Commission, 2007) Branch, Trevor A
Sightings from the IDCR/SOWER austral summer surveys are analysed to provide abundance estimates for Antarctic (true) blue whales (Balaenoptera musculus intermedia) south of 60°S. The IDCR/SOWER ship-borne surveys have completely circled the Antarctic three times: 1978/79-1983/84 (CPI); 1985/86-1990/91 (CPII); and 1991/92-2003/04 (CPIII), covering strata totalling 64.3%, 79.5% and 99.7% of the ocean surface between the pack ice and 60°S. During the surveys, blue whale sightings were rare but were recorded in all regions. Raw sighting rates (schools per 1,000 n.mile of primary search effort) were 0.44 (CPI), 0.67 (CPII) and 1.48 (CPIII). Respective circumpolar abundance estimates were 453 (CV=0.40), 559 (CV=0.47) and 2,280 (CV=0.36), with corresponding mid-years of 1981, 1988 and 1998. The CPIII estimates are the most complete and recent for this subspecies. When adjusted for unsurveyed regions in a simple way, the estimated circumpolar rate of increase is 8.2% (95% CI=1.6–14.8%) per year; nevertheless, Antarctic blue whales still number far less than the estimated 202,000-311,000 that existed before exploitation. These abundance estimates are negatively biased because some Antarctic blue whales may have been north of 60°S or in the pack ice at the time of the surveys and because a small number of blue whales on the trackline were probably missed. Furthermore, a small proportion of pygmy blue whales, probably less than 1%, may have been included in the sightings.
Open Access
Abundance of blue whales off Chile from the 1997/98 SOWER survey
(2007) Branch, Trevor A; Zerbini, Alexandre N; Findlay, Ken
The 1997/98 SOWER survey in Chile searched the region from 18°30′S to 38°S. Although the primary intention of the surveys was to maximize blue whale encounters, survey coverage was sufficient to estimate abundance using line transect methods. The baseline abundance estimate, excluding transit legs, was 452 (CV = 0.56, 95% CI: 160–1300). This abundance estimate is negatively biased because inshore regions including Chiloé Island and the Gulf of Corcovado, where blue whales are now known to aggregate, were outside the survey area. If it is conservatively assumed that the baseline estimate applied to the entire population, then the population was at a minimum of 7–23% of pre-exploitation levels in 1997.
Open Access
Analysis of simulated Antarctic minke surveys using the "standard" method and the "direct duplicate" method
(2006) Branch, Trevor A
Four new scenarios (sc33–sc36) of simulated survey data are analysed using the “standard” distance sampling method and the direct duplicate method of Palka (1995). The new scenarios were “blind” in that true simulated densities and the factors included were not revealed. Estimated densities were 0.029, 0.021, 0.082, and 0.063 whales per km 2 for the four scenarios using the “standard” method, and 0.029, 0.020, 0.081, and 0.062 whales per km 2 for the direct duplicate method. Some negative bias in estimates is expected from the standard method due to whales on the trackline being missed by the surveys. If true, the direct duplicate method then failed to correct for this bias for these scenarios, as resulting estimates were very similar to those obtained from the “standard” method.
Open Access
Applying the direct duplicate method to simulated IDCR/SOWER survey data
(2005) Branch, Trevor A; Butterworth, Doug S
The direct duplicate method (Palka, 1994) was applied to simulated IDCR/SOWER survey data. Estimates of whale density were generally negative biased, but less so than estimates obtained using the standard method. The mean bias across scenarios was -11% (range -31% to 8%) for the “2004” scenarios and -5% (range -19% to 10%) for the “2005” scenarios. Negative bias was more pronounced when a density gradient was present, when the detection function used to generate the simulated sightings excluded school size but included weather as a covariate, when errors in recorded school size were introduced, when weather and density were correlated, and when surveys were conducted in IO mode only. This method shows promise although further development is desirable to reduce the associated bias further, perhaps by including weather and school size as covariates.
Open Access
Assessment of the East Greenland-Iceland fin whale population using a four-area model
(International Whaling Commission, 2006) Branch, Trevor A; Butterworth, Doug S
The East Greenland-Iceland (EGI) fin whale population is modeled as four subpopulations with movement between the following areas: East Greenland (area 1), West Iceland (area 2), East Iceland (area 3) and the Far East (area 4). The model is sex- and age-structured, and is fitted to CPUE, sightings survey abundance, and mark-recapture data using both maximum likelihood and Bayesian approaches. Movement parameters are not differentiated by sex since the inclusion of sex-specific movement parameters did not improve the AIC. For the base case assessment scenario, best fits to the data were obtained when West Iceland and East Iceland are effectively fully mixed with a low level of interchange with East Greenland and little interchange with the Far East region. For the base case and all sensitivity tests, the overall recruited population is increasing and above 74% (base case 84%) of pre-exploitation abundance (K), and subpopulations in all areas are above 68% (base case 78%) of the individual K values. MSYR for the recruited population is 0.020 for the base case and 0.014 to 0.036 for the sensitivity tests. Projections for annual catches of 0, 100, and 200 whales taken from West Iceland indicate that only the last would result in abundance decreases compared to current levels. Under catch levels of 200 whales the probability of the total EGI population falling below 60% of pre-exploitation levels within the next 30 years was 5.7%, 7.3% and 11.5% for the 1+, recruited and mature components of the population, although there was a 51% probability of this occurring for the West Iceland mature component.
Open Access
Comparison of Sower Cpiii abundance estimates For Humpback Whales in area Iv with Jarpa abundance estimates
(International Whaling Commission, 2006) Branch, Trevor A
Estimates of abundance for humpback whales from IDCR/SOWER surveys have been provided up to and including the 1997/98 survey (Branch and Butterworth, 2001). These surveys did not cover the majority of Area IV. Data on humpback whales in Area IV were extracted from DESS for the IDCR/SOWER surveys in CPIII. Two surveys were conducted in parts of this Area: 1994/95 from 40-80°E overlapping the 70-80°E section, and the 1998/99 survey from 80-130°E covering the remainder of Area IV. Estimates were obtained separately for these two surveys. In the 1994/95 survey, the EN and ES strata covered 60- 80°E, overlapping with Area IV from 70-80°E. The Prydz Bay stratum was entirely within Area IV. There were no humpback sightings in Prydz Bay. There were only four sightings in the eastern overlapping strata. Abundance estimates for EN and ES were 53 (CV=1.01) and 84 (CV=0.53) respectively. Assuming half of the abundance was in Area IV, the total abundance in Area IV would be 68 (CV=0.51).
Open Access
Discards and revenues in multispecies groundfish trawl fisheries managed by trip limits on the U.S. West Coast and by ITQs in British Columbia
(University of Miami, Rosenstiel School of Marine and Atmospheric Science, 2006) Branch, Trevor A
The problem of multispecies fisheries, in which more productive and less productive species are caught together, is approached differently in the multispecies groundfish trawl fisheries of the U.S. West Coast and British Columbia (B.C.). In 1997 their management systems diverged: the former continued using trip limits, but the latter turned to individual transferable quotas (ITQs) combined with full observer coverage and the deduction of marketable discard mortality from quotas. U.S. requirements to rebuild overfished West Coast species have led to reduced trip limits, restrictions on fishing gear, and large area closures, which have decreased catches of species that are not overfished, increased discards of marketable fish, and decreased per-vessel groundfish income to US$220,000. In B.C., total catches have remained stable while individual incentives to retain marketable catches and to improve economic efficiency resulted in low marketable discard fractions, increased ex-vessel prices, and higher per-vessel revenue (US$420,000–US$500,000). If the B.C. system were implemented in the West Coast fishery, total revenue would probably improve through increased use of species that are not overfished, lower marketable discard fractions, and higher ex-vessel prices. Revenue increases may be hampered by restrictions imposed by the overfished species, but would probably exceed the additional observer costs.
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Escaping the tyranny of the grid: a more realistic way of defining fishing opportunities
(NRC Research Press, 2005) Branch, Trevor A; Hilborn, Ray; Bogazzi, Eugenia
A large part of fishing behavior is choosing where to fish. Trawl skippers usually choose between known fishing opportunities, which are observed as groups of trawls that are conducted in the same portion of a fishing ground, or go exploratory fishing. We outline a simple clustering method based on Euclidean distances between trawls that offers a more realistic way of defining fishing opportunities than grid cells or statistical areas. The resulting cluster tree of trawls is divided into individual groups of trawls (fishing opportunities) using a recommended cut point. Our method correctly classified simulated trawls into fishing opportunities. Fishing opportunities were obtained for vessels in the British Columbia groundfish trawl fishery; each vessel usually fished at a wide variety (mean 26, standard deviation 16, range 2 69) of fishing opportunities. Within each fishing opportunity, trawls generally caught similar species. In the Argentina scallop fishery, our method was able to divide exploratory from regular fishing trawls, with obvious applications for catch-per-unit-effort calculations. Our method could also be used to detect positional errors in data from these fisheries. Fishing opportunities could provide indications of how fishermen might react to marine protected areas and to the imposition of quotas on multispecies fisheries.
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Estimated density of Antarctic minke whales obtained from simulated IDCR/SOWER survey data using the “standard” method
(2005) Branch, Trevor A
Simulated survey datasets provided by Palka and Smith were analysed using standard line transect methods that assume g(0)=1. Estimates of mean school size were generally positively biased by >10% when cue production was asynchronous, but otherwise typically unbiased. Overall estimates of whale density were negatively biased for nearly all scenarios considered, with mean bias of -23% for “2004” scenarios and -10% for “2005” scenarios. In the “2004” scenarios, bias was greatest when the detection function used to generate the simulated sightings included school size or weather as a covariate; in the “2005” scenarios, the greatest bias occurred when weather and whale density were correlated and when survey was in IO mode only. Implied values of g(0) ranged from 0.48 to 1.02 (mean 0.77) for the “2004” scenarios and from 0.78 to 1.03 (mean 0.90) for the “2005” scenarios, and were higher than previous estimates from the IDCR surveys (generally 0.5–0.7).
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Evidence for increases in Antarctic blue whales based on Bayesian modelling
(Wiley, 2004) Branch, Trevor A; Matsuoka, Koji; Miyashita, Tomio
Antarctic blue whales (Balaenoptera musculus intermedia) are the largest and formerly most abundant blue whale subspecies, but were hunted to near extinction last century. Estimated whaling mortality was unsustainable from 1928 to 1972 (except during 1942–1944), depleting them from 239,000 (95% interval 202,000–311,000) to a low of 360 (150–840) in 1973. Obtaining statistical evidence for subsequent increases has proved difficult due to their scarcity. We fitted Bayesian models to three sighting series (1968–2001), constraining maximum rates of increase to 12% per annum. These models indicated that Antarctic blue whales are increasing at a mean rate of 7.3% per annum (1.4%–11.6%). Informative priors based on blue whale biology (4.3%, SD = 1.9%) and a Bayesian hierarchical meta-analysis of increase rates in other blue whale populations (−3%, SD = 11.6%), suggest plausible increase rates are lower (although the latter has wide intervals), but a meta-analysis of other mysticetes obtains similar rates of increase (6.7%, SD = 4.0%). Possible biases affecting the input abundance estimates are discussed. Although Antarctic blue whales appear to have been increasing since Sovier illegal whaling ended in 1972, they still need to be protected-their estimated 1996 population size, 1,700 (860–2,900), was just 0.7% (0.3%–1.3%) of the pre-exploitation level.
Open Access
Female length at sexual maturity for pygmy and Antarctic blue whales based on Soviet ovarian corpora, 1961-72
(2007) Branch, Trevor A; Mikhalev, Y A
Female blue whale ovarian corpora data were translated and encoded from the USSR’s Slava (1961/62–1965/66) and Sovietskaya Ukraina (1961/62–1971/72) expeditions. Complete ovarian data were available for 1,425 blue whales (1,272 pygmy, 153 Antarctic). Catches north of 52°S were assumed to be pygmy blue whales (Balaenoptera musculus brevicauda), while those south of 56°S were assumed to be Antarctic (true) blue whales (B. m. intermedia), although there was some evidence for a small proportion (<1%) of both Antarctic blue whales north of 52°S and pygmy blue whales south of 56°S. A small proportion of lengths were rounded to the nearest metre, and many whales shorter than 18.0 were recorded as 18.0 m or greater (whale stretching). A Bayesian logistic model fitted to the data provided estimates of L50 and L95 (the lengths at which 50% and 95% of females are sexually mature). For pygmy blue whales L50 was 19.2 m (95% interval 19.1–19.3 m) and L95 was 20.5 m (95% interval 20.4–20.7 m). These estimates are more precise than those from Japanese data because the Soviet vessels recorded 32 times more pygmy blue whales shorter than the legal minimum length (21.3 m). Among small areas, L50 varied from 18.4 to 19.9 m for pygmy blue whales; all estimates were much shorter than the 23.4 m from the Antarctic. The status of northern Indian Ocean pygmy blue whales is unclear: L50 for these blue whales was statistically significantly shorter than L50 for both the southern Indian Ocean and around Australia, but the magnitude of the differences was small: 0.5–0.6 m.
Metadata only
Fleet dynamics and behavior:lessons for fisheries management
(NRC Research Press, 2011) Branch, Trevor A; Hilborn, Ray; Haynie, Alan C; Fay, Gavin; Flynn, Lucy; Griffiths, Jennifer; Marshall, Kristin N; Randall, Jeffrey K; Scheuerell, Jennifer M; Ward, Eric J; Young, Mark
We review fleet dynamics and fishermen behavior from an economic and sociological basis in developing fisheries, in mature fisheries near full exploitation, and in senescent fisheries that are overexploited and overcapitalized. In all cases, fishing fleets behave rationally within the imposed regulatory structures. Successful, generalist fishermen who take risks often pioneer developing fisheries. At this stage, regulations and subsidies tend to encourage excessive entry and investments, creating the potential for serial depletion. In mature fisheries, regulations often restrict season length, vessel and gear types, fishing areas, and fleet size, causing or exacerbating the race for fish and excessive investment, and are typically unsuccessful except when combined with dedicated access privileges (e.g., territorial rights, individual quotas). In senescent fisheries, vessel buyback programs must account for the fishing power of individuals and their vessels. Subsidies should be avoided as they prolong the transition towards alternative employment. Fisheries managers need to create individual incentives that align fleet dynamics and fishermen behavior with the intended societal goals. These incentives can be created both through management systems like dedicated access privileges and through market forces.
Open Access
Humpback whale abundance south of 60°S from three complete circumpolar sets of surveys
(International Whaling Commission, 2011) Branch, Trevor A
Austral summer estimates of abundance are obtained for humpback whales (Megaptera novaeangliae) in the Southern Ocean from the IWC’s IDCR and SOWER circumpolar programmes. These surveys have encircled the Antarctic three times: 1978/79–1983/84 (CPI), 1985/86–1990/91 (CPII) and 1991/92–2003/04 (CPIII), criss-crossing strata totalling respectively 64.3%, 79.5% and 99.7% of the open-ocean area south of 60°S. Humpback whales were absent from the Ross Sea, but were sighted in all other regions, and in particularly high densities around the Antarctic Peninsula, in Management Area IV and north of the Ross Sea. Abundance estimates are presented for each CP, for Management Areas, and for assumed summer feeding regions of each Breeding Stock. Abundance estimates are negatively biased because some whales on the trackline are missed and because some humpback whales are outside the survey region. Circumpolar estimates with approximate midpoints of 1980/81, 1987/88 and 1997/98 are 7,100 (CV = 0.36), 10,200 (CV = 0.30) and 41,500 (CV = 0.11). When these are adjusted simply for unsurveyed northern areas, the estimated annual rate of increase is 9.6% (95% CI 5.8–13.4%). All Breeding Stocks are estimated to be increasing but increase rates are significantly greater than zero only for those on the eastern and western coasts of Australia. Given the observed rates of increase, the current total Southern Hemisphere abundance is greater than 55,000, which is similar to the summed northern breeding ground estimates (~60,000 from 1999–2008). Some breeding ground abundance estimates are far greater, and others far lower, than the corresponding IDCR/SOWER estimates, in a pattern apparently related to the latitudinal position of the Antarctic Polar Front.
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Past and present distribution, densities and movements of blue whales Balaenoptera musculus in the Southern Hemisphere and northern Indian Ocean
(Wiley, 2007) Branch, Trevor A; Stafford, K M; Palacios, D M; Allison, C; Bannister, J L; Burton, C L K; Cabrera, E; Carlson, C A; Galletti Vernazzani, B; Gill, P C; Hucke-Gaete, R; Jenner, K C S; Jenner, M N M; Matsuoka, K; Mikhalev, Y A; Miyashita, T; Morrice, M G; Nishiwaki, S; Sturrock, V J; Tormosov, D; Anderson, R C; Baker, A N; Best, P D; Borsa, P; Brownell Jr, R I; Childerhouse, S; Findlay, K P; Gerrodette, T; Ilangakoon, A D; Joergensen, M; Kahn, B; Ljungblad, D K; Maughan, B; Mccauley, R D; Mckay, S; Norris, T F; Oman Whale and Dolphin Research Group; Rankin, S; Samaran, F; Thiele, D; van Waerebeek, K; Warneke, R M
1. Blue whale locations in the Southern Hemisphere and northern Indian Ocean were obtained from catches (303 239), sightings (4383 records of 8058 whales), strandings (103), Discovery marks (2191) and recoveries (95), and acoustic recordings. 2. Sighting surveys included 7 480 450 km of effort plus 14 676 days with unmeasured effort. Groups usually consisted of solitary whales (65.2%) or pairs (24.6%); larger feeding aggregations of unassociated individuals were only rarely observed. Sighting rates (groups per 1000 km from many platform types) varied by four orders of magnitude and were lowest in the waters of Brazil, South Africa, the eastern tropical Pacific, Antarctica and South Georgia; higher in the Subantarctic and Peru; and highest around Indonesia, Sri Lanka, Chile, southern Australia and south of Madagascar. 3. Blue whales avoid the oligotrophic central gyres of the Indian, Pacific and Atlantic Oceans, but are more common where phytoplankton densities are high, and where there are dynamic oceanographic processes like upwelling and frontal meandering. 4. Compared with historical catches, the Antarctic (‘true’) subspecies is exceedingly rare and usually concentrated closer to the summer pack ice. In summer they are found throughout the Antarctic; in winter they migrate to southern Africa (although recent sightings there are rare) and to other northerly locations (based on acoustics), although some overwinter in the Antarctic. 5. Pygmy blue whales are found around the Indian Ocean and from southern Australia to New Zealand. At least four groupings are evident: northern Indian Ocean, from Madagascar to the Subantarctic, Indonesia to western and southern Australia, and from New Zealand northwards to the equator. Sighting rates are typically much higher than for Antarctic blue whales.
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Practical application of meta-analysis results: avoiding the double use of data
(NRC Research Press, 2005) Minte-Vera, C V; Branch, Trevor A; Stewart, I J; Dorn, M W
Meta-analysis is an important new tool for synthesizing scientific knowledge from many previous studies. In fisheries, meta-analyses can be used to obtain prior distributions or penalty functions for parameters used in stock assessment models. Two types of results are generally published in a meta-analysis: Type A, the updated results for each stock used in the meta-analysis, and Type B, the results that would best describe a new stock. Including these results in assessments for the individual stocks would result in double use of the data if the assessments include the input data used in the meta-analyses, which they typically would. To solve this problem, we recommend that an additional form of results should be reported in meta-analyses: Type C, the results for a new stock obtained by sequentially excluding each stock's data set and repeating the meta-analysis. Type C results should be used whenever the assessment input data overlap with the meta-analysis input data, avoiding the double use of data. We illustrate the impact of this reporting change on the results of a recent meta-analysis.
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Preliminary abundance estimates for Antarctic minke whales from three completed sets of IDCR/SOWER circumpolar surveys, 1978/79 to 2003/04
(2005) Branch, Trevor A
Abundance estimates are provided for Antarctic minke whales from the ship-based IDCR-SOWER surveys using standard distance sampling methodology. The methods of pooling strata and of estimating mean school size have altered since the most recent re-assessment of all these surveys. The IDCR-SOWER surveys are grouped into three completed circumpolar sets of cruises: 1978/79–1983/84 (CPI), 1985/86–1990/91 (CPII) and 1991/92–2003/04 (CPIII), which respectively covered 63.1%, 79.5% and 99.9% of the ice-free area south of 60°S. Abundance estimates from individual surveys were added for CPI and CPII and combined using the ‘surveyonce’ method for CPIII to obtain circumpolar abundance estimates. When closing and independent observer mode estimates were inverse-variance weighted, circumpolar abundance estimates were 594,000 (CV = 0.128), 769,000 (CV = 0.094) and 362,000 (CV = 0.080) for CPI, CPII and CPIII respectively. These estimates are known to be negatively biased because some Antarctic minke whales may be north of 60°S or inside the pack ice during the surveys, and because some whales on the trackline were likely missed. After simple extrapolation to account for differences in the latitudes surveyed during each circumpolar set (but not for increasing proportions of ‘like minke’ sightings), the ratio of the circumpolar estimates was 0.92:1.00:0.39, echoing previous findings of appreciably lower CPIII estimates. These results should be considered preliminary.
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Regional differences in length at sexual maturity for female blue whales based on recovered Soviet whaling data
(Society for Marine Mammalogy, 2008) Branch, Trevor A; Mikhalev, Y A
New blue whale ovarian corpora data from illegal Soviet catches in the Southern Hemisphere and northern Indian Ocean were recovered from the original logbooks. Catches north of 52°S were assumed to be pygmy blue whales (Balaenoptera musculus brevicauda, n= 1,272); those south of 56°S were assumed to be Antarctic (true) blue whales (B. m. intermedia, n= 153). Three probable Antarctic blue whales north of 52°S were excluded. Lengths at which 50% and 95% of females become sexually mature (L50 and L95) were estimated from a Bayesian logistic model. These estimates are more precise than previous Japanese estimates because Soviet catches below the legal minimum of 70 ft (21.3 m) were 32 times greater. For pygmy blue whales L50 was 19.2 m (95% interval 19.1–19.3 m) and L95 was 20.5 m (95% interval 20.4–20.7 m). Antarctic L50 (23.4 m, 95% interval 22.9–23.9 m) was much longer than L50 for pygmy blue whale regions (18.4–19.9 m). The median L50 for the northern Indian Ocean was 0.5–0.6 m shorter than for pygmy blue whales from other regions; although statistically significant, these small length differences provide little support for northern Indian Ocean blue whales being a separate subspecies, B. m. indica.
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Replacing trip limits with individual transferable quotas:implications for discarding
(Elsevier, 2006) Branch, Trevor A; Rutherford, Kate; Hilborn, Ray
In the British Columbia groundfish fishery (BC fishery), full observer coverage and the accounting of discard mortality of marketable fish in landing limits resulted in low discard fractions. When individual transferable quotas (ITQs) were additionally introduced, total discard fractions declined for most species, and marketable discard fractions declined from 0.20% to 0.10%, after an adjustment period. In contrast, the US West Coast groundfish fishery, which is regulated by 2-monthly landing limits, has higher discard fractions (31–43% vs. 14–19% for BC). The BC experience suggests that full observer coverage, ITQs, and mortality accounting would reduce West Coast discarding fractions, but severe catch restrictions on overfished West Coast species may limit such reductions.
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A scientific alternative to moratoria for rebuilding depleted international tuna stocks
(Wiley, 2015) Hillary, Richard M; Preece, Ann L; Davies, Campbell R; Kurota, Hiroyuki; Sakai, Osamu; Itoh, Tomoyuki; Parma, Ana M; Butterworth, Doug S; Ianelli, James; Branch, Trevor A
There is considerable international concern and scientific debate about the current state and future of tuna stocks worldwide and the capacity of Regional Fisheries Management Organisations to manage the associated fisheries effectively. In some cases, this concern has extended to predictions of imminent collapse with minimal chances of recovery, even under a commercial catch moratorium. As a viable alternative to a full fishery closure, the Commission for the Conservation of Southern Bluefin Tuna (CCSBT) has adopted a scientifically tested, adaptive rebuilding strategy for the depleted southern bluefin tuna (Thunnus maccoyii) stock. The management procedure (MP) adopted involves a harvest control rule that fully specifies the total allowable catch as a function of key indicators of stock status, adjusting future harvest levels every three years so as to meet the rebuilding targets agreed by CCSBT. It was chosen from a subset of candidate MPs selected following extensive simulation testing. This involved first selecting a wide range of plausible scenarios for stock status and input data, ranging from pessimistic to optimistic, against which the alternative candidate MPs were tested to ensure that they were robust to important uncertainties. This is the first time that a comprehensively evaluated MP has been adopted for an internationally managed tuna stock. Both the process and the outcomes have broad applicability to other internationallymanaged stocks.