Thumbnail Image

Report of the training course on single-species length based stock assessment methods

Kavala, Greece, 21 - 25 February 2011








Also available in:
No results found.

Related items

Showing items related by metadata.

  • Thumbnail Image
    Book (stand-alone)
    Introduction to tropical fish stock assessment - Part 1: Manual (French version not published) 1998
    Also available in:

    In Part 1, Manual, a selection of methods on fish stock assessment is described in detail, with examples of calculations. Special emphasis is placed on methods based on the analysis of length-frequencies. After a short introduction to statistics, it covers the estimation of growth parameters and mortality rates, virtual population methods, including age-based and length-based cohort analysis, gear selectivity, sampling, prediction models, including Beverton and Holt's yield per recruit model and Thompson and Bell's model, surplus production models, multispecies and multifleet problems, the assessment of migratory stocks, a discussion on stock/recruitment relationships and demersal trawl surveys, including the swept-area method. The manual is completed with a review of stock assessment, where an indication is given of methods to be applied at different levels of availability of input data, a review of relevant computer programs produced by or in cooperation with FAO, and a list of refer ences, including material for further reading. In Part 2, Exercises, a number of exercises is given with solutions. The exercises are directly related to the various chapters and sections of the manual.
  • Thumbnail Image
    Meeting
    Provision of scientific advice for the purpose of the implementation of the EUPOA sharks. Final Report. European Commission, Studies for Carrying out the Common Fisheries Policy (MARE/2010/11 - LOT 2) 2013
    Also available in:
    No results found.

    The scope of the European Union Plan of Action for Sharks covers directed commercial, by-catch commercial, directed recreatiol, and by-catch recreatiol fishing of any chondrichthyans within European Union waters. It also includes any fisheries covered by current and potential agreements and partnerships between the European Union and third countries, as well as fisheries in the high seas and fisheries covered by RFMOs maging or issuing non-binding recommendations outside European Union waters. S cientific advice for the purpose of the magement of shark species in the high seas is carried out mainly via the Scientific Committees of the relevant Regiol Fisheries Magement Organisations (RFMO), as well as through specific projects by tiol institutes, and other research organisms. However, the level of knowledge concerning many shark populations in the high seas of the Atlantic, Indian and Pacific Oceans is far from satisfactory. It is therefore necessary to identify gaps in the current know ledge of fisheries, biology and ecology of sharks that should be filled in order to support advice on sustaible magement of elasmobranches' fisheries and undertaking studies to fill those gaps. Therefore, the objective of this project is to obtain scientific advice for the purpose of implementing the EUPOA on sharks as regards the facilitation of monitoring fisheries and shark stock assessment on a species-specific level in the high seas. The study is focused on major elasmobranch species caught by both artisal and industrial large pelagic fisheries on the High Seas of the Atlantic, Indian and Pacific area, which are currently monitored and potentially maged by respective Tu RFMOs. Specifically, firstly the study aims to collate and estimate historical fisheries data especially on species composition of catches, catches and effort, size frequencies in order to identify the gaps in the current availability of fishery statistics as well current knowledge biology and ecology of sharks tha t should be filled in order to support the scientific advice provided to RFMOs on sustaible magement of elasmobranch fisheries. And secondly, the project aims to review and prioritise the gaps identified to develop a research program to fill those gaps in support for the formulation of scientific advice for magement of sharks. The data and knowledge gaps identified through Phase I will allow focusing and prioritising the future research. From this summarisation of Phase I it will be clear as to what data is available for providing magement advice for shark species, and where gaps in the data render this task difficult. In a second step, recommendations for data collection improvements as well as research necessities and activities will be described. The data collected in Phase I of the project gives a complete picture of the current data availability of information about catch and effort, observer programs, size frequency information, biological information and fishery indicators that may support the assessment of major shark species in Tu RFMOs. In spite of the importance of shark catches by industrial fleets, they have traditiolly consisted of bycatch of commercial fisheries and sharks are most often discarded or finned. Therefore, most of the times, shark catches are not recorded, especially with the required level of resolution, and catches must be estimated by statistical procedures based on observer data, fishing effort and different covariates. Moreover, the informatio n recorded is not usually RFMO) databases. The information on shark bycatch is scarce and their estimates found in the literature are not homogenous which made the raising and/or estimates of ratios (by-catch/target catch) uncertain due to various assumptions made (e.g. conversion of the estimates in number of individuals into weight without any information on the mean size per species). In summary, the main difficulties and data gaps identified in the project can be described as follows: there is a lack of shark reporting in artisal and coastal fisheries; there is a lack of shark reporting in industrial fisheries and when is reported usually is not broken down by species; there is a lack of any size frequency data; there is a lack of regiol biological/ecological information for sharks; there are difficulties to access to the data both at RFMO level and at a country level; species misidentification which affect the collection of fishery statistics; low observer coverage for most of the fleets/areas; difficulties with the use of logbook data for shark assessment (misidentification, underreporting, change in targeting practice). The work carried out in first phase of the project focuses on the collection of fishery information publicly available, mainly in the website public domain of the RFMOs in the Indian, Pacific and Atlantic Ocean as well as in the Mediterranean Sea (ICCAT- GFCM, IOTC, IATTC, and WCPFC) but also on information available in the literature, most of which com es from documents presented at the scientific meetings or workshops of RFMOs. The RFMO data administrators were also contacted in order to obtain any additiol fishery statistics data. Similarly, information from flag states, and from EU- member states, was requested in order to improve the information available on discards levels, size frequencies and biological information. Apart from RFMO official statistics, and in order to get more accurate and altertive catch data, shark catch estimations f or the most recent period were appraised based on fleet specific ratios of shark catch over tu (or target) catches. This was done in a two step process, first a general ratio between shark catches over tu (target) catch was applied to estimate total shark catches for major fisheries and, then, the relative proportion by species in the catch was applied to estimate shark catches by species. Those ratios were obtained from the literature search and/or data from observer programs available in the R FMO or in the literature. This exercise allows identifying the fleets that could be mainly responsible for the catch of the main shark species included in the study based on the best assumption of the shark catch over target species catch ratios derived from the literature but also allows identifying the main impacted shark specis by fisheries in each areas as well as the main origin of underreporting. In the Atlantic and Pacific (east and west), the Longline fleet targeting sharks, swordfish an d/or tropical tus is the most important métier catching sharks; which contributes with 59 %, 86 % and 95 % to the total shark catches respectively. On the contrary, the picture in the Indian Ocean is different where gillnet (GN - sensu lato) are contributing with 61 % of the total shark catch in comparison to 18 % for longliners. In general, the species composition of the sharks in different métiers is similar in all Oceans as well as in the Mediterran Sea. For example, Longline (LL - sensu lato ) impacts mainly blueshark and shortfin mako and in a minor extend hammerhead, thresher, silky and oceanic whitetip sharks; whereas Gillnet (GN - sensu lato) are impacting mainly silky, thresher, Oceanic whitetip, and shortfin mako sharks. The catch of silky and oceanic whitetip shark for the longline fleet in the West Pacific is higher than other longline fleets of other Oceans because they are operating in more equatorial waters. Although, in all the Oceans, the contribution to the total catch of Purse seines is minor (maximum of 5 % of total catch in the West Pacific); the species composition of purse seines catch is clearly domited by silky and oceanic whitetip sharks. In all Oceans the main species impacted is blueshark with around 65-75 %, with the exception of the Indian Ocean and Mediterranean Sea, of the total shark catch. The contribution of the rest of the species can vary depending on the relative contribution of different fleets as well as the spatial distribution of the d ifferent fleets. However, in general the blueshark catch is then followed by shortfin mako, hammerhead, silky, thresher, Oceanic withetip shark. In the Indian Ocean, the blueshark contribution to total shark catch is around 35 % followed by silky shark (21 %), thresher (16 %), Oceanic whitetip (11 %), shortfin mako (10 %) and hammerheads (6 %). And in the Mediterrenean, blueshark contribution is around 50 % while other species make up the rest: thresher sharks (25 %), mako sharks (13.3 %), tope shark (6.1 %), rays (3.5 %), and porbeagle (1 %). The comparison between the declared value and the estimated value can be considered as a figure for undereporting. For example, it is worth mentioning that the total average amount of the investigated species estimated is 1.5, 13 and 7 times higher than the average amount declared in the Atlantic Ocean, Mediterranean Sea and Indian Ocean, respectively. However, as the estimation carried out in this alysis was based on ratio of shark catch over to tal target catch there is high uncertainty on fil estimations coming from different sources; such as métier classification, from target species quantities declared and from the shark/target catch ratio used to estimate the shark species investigated; which recommend to take these estimations with caution. It was not possible to apply this methodology to the Eastern and Western Pacific due to the lack of access to disaggretate tu/target species catches from the IATTC/WCPFC public databases. And t he data above should be considered in the light of the different species productivity and susceptibility of a given species to a giving métier. This is important to take into account because in some cases a minor catch of one species from all fleets, or one fleet in particular, can have a great impact if the species in question is more vulnerable showing low productivity and high susceptibility to the fleet(s). So, it is important to consider the results above in the framework of Ecological Risk Assessment (ERA) which can help to identify priorities for observer programs/research efforts. Filly, data gaps identified in relation to shark fishery statistics have been summarized with the aim to develop a research framework that would allow filling those gaps in order to assess and mage the shark population worldwide in a sustaible manner. The design of such programme is benefited and integrates all the information collected through phase I. For example, the data and knowledge gaps identif ied and listed/inventoried through Phase I allows focusing and prioritised the future research. From this summarisation of Phase I it is clear as to what data is available for providing magement advice for shark species, and where gaps in the data render this task difficult. At this stage, recommendations for data collection improvements as well as research necessities and activities are described. As such, the review of existing information; as well as the identification of information gaps, ma in shark species impacted and main métier responsible for major shark catch; presented above provides the basis for development of a research program and priorities for the assessment of the status of sharks in Tu RFMOs. As it is not possible to develop a research program for all the Tu RFMOs, a general framework to develop the research program in support of the scientific advice for shark magement is proposed; which includes: (1) a research framework to identify the main species and fleets that needs to be prioritized for the collection of fishery data and information in order to assure the assessment of principal shark species regiolly in the Tu RFMOs; (2) a general recommendations for all Tu RFMOs to improve the data collection to fill the gaps identified above; and (3) options for magement and mitigation measures for sharks. The research framework is proposed to be organized in three steps: (i) estimation of shark catches by species using the method proposed here which allows ident ifying the most impacted shark species and the métier most affecting those species; (ii) a prelimiry Ecological Risk Assessment (or other prelimiry assessment based on fishery indicators) by fleets which allows to identify the most vulnerable species to focus the efforts in conjunction with point (i); and (iii) specific recommendations of how to apply possible magement measures, to improve data collection and assessment of those fleets/species identified as priorities based on points (ii) and (i ii). The implementation of the three steps is highly related. The project also recommends actions to fill the identified gaps structured in sections as data collection, data reporting, data resolution, data access, and assessment. As the data collected through phase I give a complete picture of what are the main fleets targeting the more important shark species caught in the Tu RFMOs, both EU and other countries catching shark, as well as the extent of their volume; this exercise also helps to i dentify the different species for which more focus is needed and those that are supposed to be caught in a lesser extent. For example, this helps to focus the target or more important fleets to monitor and design specific representative observer schemes for those fleets as necessary. Having in mine the data gaps for major fisheries impacting pelagic sharks stocks in the different t-RFMOs Conventions areas as well as the most important metier catching sharks and most impacted shark species; the p roject proposes some possible solutions and recommendations for the implementation of observers programmes on those fisheries, aiming to improve shark data collection, mely regarding shark catch and discards: species composition; vessel mortality; size and sex data. Magement measures are essential when a given stock is seriously affected by the fishing activity and are aimed at limiting the impact of this activity. The election of a measure will depend on the stock status, on the behavior of the species, on the species being target or not, etc.; but the project summarizes several options of magement and mitigation measures applicable to shark species.
  • Thumbnail Image
    Project
    Report on the regional training course on fish stock assessment 1991
    Also available in:
    No results found.

    Chambo is a species complex consisting of three tilapine cichlids (Oreochromis lidole, O. quamipinnis, O. karongae) endemic to Lake Malawi. Taxonomic identification is problematic at the young stages of the fishes. Chambo constitute the basis of an important commercial and artisanal fishery. Length frequency data were collected three times per month from a commercial midwater trawl. The von Bertalanffy growth parameters obtained by the Bhattacharya method and Gulland and Holt plot are cons idered not valid according to the authors knowledge of the species biology and fishery. Then a set of growth and natural mortality parameters was forced to proceed into the analysis. Catch curve method was applied to achieve an estimate of the total mortality. Virtual population analysis (Jones' length cohort analysis)- shows that only the large size fish are exposed to the fishing mortality. It was also stressed that there is need for more representative samples to get more meaningful res ults. Experimental bottom trawl-survey was carried out between 1981-1986 in the Ugandan side of Lake Victoria. Daily commercial catches from various fish landings were available. The individual length and weight data set used in the analysis was from Jana-Nsadzi area of the lake. Data have been collected in June, September, November and December 1981. Batthacharya method and Gulland and Holt plot were used to estimate the growth parameters. The catch curve analysis supplied the total morta lity value. Jones' length cohort analysis was applied to obtain the exploitation pattern. Finally long term prediction was carried out using the Thompson and Bell model. The total yield and the calculated Maximum Sustainable Yield (MSY) for the Ugandan side of the lake seem to suggest the overexploitation of Synodontis victoriae. This scenario is considered unrealistic.

Users also downloaded

Showing related downloaded files

No results found.