Fisheries stock sustainability
There are now a number of 'seafood chooser' initiatives globally that seek to inform the consumer on sustainable seafood choices. They differ in a number of ways, with some being voluntary such as the Marine Stewardship Council (MSC), while others are mandatory, such as the FRDC’s Status of Australian Fish Stocks and the United Nations’ Food and Agriculture Organisation’s Fishery Status Reporting Process. There is also notable variability in terms of scope and rigor (a ranking of quality is provided by GSSI, https://www.ourgssi.org/ ). But all generally involve assessment against pre-set requirements.
The FRDC’s Status of Australian Fish Stocks (SAFS) reports seeks to provide advice on fisheries stock sustainability, based on evidence and guidelines that are transparently reported. Minderoo’s Flourishing Oceans has also developed the Global Fishing Index (GFI), which aims to synthesise the health of fish stocks and state of fisheries governance in maritime countries around the world.
Importantly, SAFS considers only stock status, whereas the GFI includes a stock status component and also assesses the broader governance context for fish stocks, including dimensions such as stakeholder participation, compliance management and control of access.
The first edition of Minderoo’s GFI will be published in the second half of 2021, and covers more than 140 countries, including Australia. The reported performance of Australia’s fisheries under the GFI will likely differ from that in SAFS. Differences will in part reflect dissimilarities in scope and focus between the two products. More significantly, however, differences between the stock status component of the GFI and stock status as reported in SAFS will reflect dissimilar use of reference points in the two products.
What is a reference point?
In fisheries science and management, reference points are 'the operational or measurable benchmarks that identify targets to be achieved on average, limits to be avoided, and triggers to initiate management responses' (Sainsbury 2008). Reference points are usually set in relation to fish abundance (biomass), fishing mortality (i.e. removals from the population due to fishing) or both. Best-practice management generally involves setting reference points for both biomass and fishing mortality. This is because the biomass depletion gives the ultimate impact on the population of fishing plus non-fishing variability and change (e.g. environmental variability or change) while fishing mortality is under direct management control (Sainsbury 2008). For example, common patterns of recruitment variability can result in wide ranges of biomass even if fishing is perfectly controlled to give maximum long-term yield (Hilborn 2020). For commercially exploited fish stocks, the following types of reference points are most commonly used:
- Target reference point: the point around which biomass and/or fishing mortality would ideally fluctuate.
- Limit reference point: a low (for biomass) or high (for fishing mortality) value that the stock should not fall below. Management should be such that there is a very low likelihood (e.g. <10 percent) of breaching a limit reference point. Immediate management intervention is required if a limit reference point is breached.
- Trigger reference points: typically intermediate between target and limit points, these provide the cue to initiate management responses to declining biomass and/or rising fishing mortality to bring the stock back towards the target reference point.
What are common reference points?
Reference points should be set according to clearly defined management objectives for a stock (Hilborn 2020). Management objectives for a stock could be maximising sustainable food production, maximising economic returns from fishing or maintaining a stock that is important for broader ecosystem function (e.g. important prey species) at a precautionary biomass level (Haddon et al. 2013, Hilborn et al. 2020). Each of these objectives would likely require different reference points values but, in each case, there is also a fundamental need to protect the biological productivity through avoiding the limit reference point.
Best practice in setting reference points should also consider a stock’s key life history and demographic traits (e.g. longevity, fecundity, growth rates, age at maturity). As a generalisation, species that are short-lived, fast growing and highly fecund could have lower reference points for biomass and higher reference points for fishing mortality than those that grow slowly, mature later in life and produce fewer offspring. Nonetheless, fast-growing, short-lived species, even when highly fecund, are typically subject to large natural variations in abundance, even in the absence of fishing, and their populations can collapse rapidly if excessive fishing pressure coincides with a period of naturally low abundance or poor recruitment (Haddon et al. 2013). In some cases, the change in the environmental or stock conditions can result in the need for dynamic reference points that reflect current circumstances. And sometimes it is appropriate to change the indicator being tracked, as for example in some small pelagic fisheries where egg production replaces the biomass indicator, but a reference point is still required for the new indicator.
Additionally, stocks of key prey or forage species in marine ecosystems may require conservative reference points to maintain ecosystem integrity. For example, a very high biomass reference point of 80 per cent of unfished biomass may be appropriate for small pelagic fish species to protect predator populations, even though the small pelagic species typically have traits bestowing resilience to fishing pressure (Haddon et al. 2013).
Despite these arguments for tailoring reference points to a particular species (or even better, a particular stock), there are some general levels of both biomass and fishing mortality that are widely used as reference points (typically through a proxy value). Maximum Sustainable Yield (MSY) describes the level at which consistent extraction delivers a sustained maximised yield (Punt et al. 2013), and the biomass and fishing mortality that would deliver MSY over an extended period (denoted BMSY and FMSY respectively) are often selected as target reference points. Varyingly conservative percentages of BMSY and FMSY may also be used as a target reference point to acknowledge precaution and natural variations in stock abundance (Hilborn et al. 2020). Across a range of possible stock-recruitment relationships (which in any case are generally poorly understood in real stocks), a target reference point of 35–40 percent of unfished biomass proved robust where the management objective was to maximise yield (Punt et al. 2013).
Similarly, but more conservatively, the biomass and fishing mortality rate that would maximise economic return (Maximum Economic Yield, MEY) may also be selected as target reference points. Forty-eight percent of unfished biomass is sometimes taken as a proxy for MEY, and modelling indicates that depletion levels in the range of 50–70 percent of unfished biomass are suitable across a broad range of stock-recruitment relationships where the aim is to maximise profits (Punt et al. 2013).
Limit reference points tend to be set around the point at which irreversible (or slowly reversible) recruitment impairment would occur. Twenty percent of unfished biomass is often used as a limit reference point and may be appropriate (or even conservative) for some highly productive stocks, but insufficiently conservative for other slow-growing, long-lived, and/or late-maturing stocks (Sainsbury 2008, Haddon et al. 2013). Again, however, the point of recruitment impairment for a given stock will vary according to its life history and demographic traits, as well as environmental and ecological variables that change through time.
The Food and Agriculture Organization of the United Nations (FAO) for status reporting offers a useful comparison of a commonly used approach for classification of biomass depletion and may also align with the approach used in the GFI. Within the FAO reporting framework, a stock is considered to be:
- ‘overexploited’ if present biomass is found to be <40 percent of unfished biomass
- ‘fully exploited’ if present biomass is between 40 and 60 percent of unfished biomass, and
- ‘underexploited’ if present biomass is >60 percent of unfished biomass.
Reference points in SAFS
The SAFS stock status framework is portrayed diagrammatically below:
Key points regarding reference points in the SAFS framework are:
- SAFS reports primarily use limit reference points. Some jurisdictions that use target reference points refer to them in their SAFS stock status sections, but they are not currently an explicit component of the stock status classification process in SAFS. Potential incorporation of target reference points in SAFS will be subject to ongoing discussion with the SAFS Advisory Group.
- If SAFS did use target reference points, these would likely lie around the middle of the green 'sustainable' domains (for example, the categorisation used by FAO status reporting).
- Each SAFS status is a composite consideration of biomass and fishing mortality; for example, a 'sustainable' status requires evidence that both biomass and fishing mortality are above the limit reference point.
- The SAFS framework does not define particular 'one size fits all' levels of biomass or fishing mortality as limit reference points. Rather, the biomass limit reference point is defined as the biomass below which recruitment impairment is likely, with the fishing mortality limit reference point similarly defined as the mortality rate that would drive the stock towards this point. Jurisdictional author teams then work together to define these points for each stock. Some SAFS assessments do use 20 percent of unfished biomass as a limit reference.
Stock status and reference points in the GFI
The biological sustainability component of the GFI focuses only on biomass and does not explicitly consider the fishing mortality rate experienced by a stock. Importantly, the GFI uses 40 percent of unfished biomass as a measure of sustainability; presumably, if the GFI had an explicit target reference point, it would be greater than 40 percent of unfished. In the GFI methodology, stock abundance is estimated from official stock assessments and, where these are not available, novel estimates are generated using data-limited assessment methods. Biomass estimates from stock assessments conducted since 2015 are used directly, whereas those from assessments earlier than 2015 are used as 'model priors' to generate contemporary estimates in combination with other data or analyses. Biomass estimates are then combined with information on data reliability and availability (i.e. proportion of total national catch that comes from assessed stocks) to provide an overall biological sustainability rating.
The GFI also uses reconstructed catch data sets from the Sea Around Us program (www.searoundus.org—note, this link is not secure). The methodology by which these reconstructions are achieved is unclear.
Haddon, M., Klaer, N., Smith, D., Dichmont, C. & Smith, T. (2013). Technical Reviews for the Commonwealth Harvest Strategy Policy. FRDC 2012/225. CSIRO, Hobart, 69 pp.
Hilborn, R. (2020). Measuring fisheries management performance. ICES Journal of marine Science doi:10.1093/icesjms/fsaa119
Punt, A.E., Smith, A.D.M., Smith, D.C., Tuck, G.N. & Klaer, N. (2013). Selecting relative abundance proxies for BMSY and BMEY. ICES Journal of Marine Science doi:10.1093/icesjms/fst162
Sainsbury, K. (2008). Best practice reference points for Australian fisheries. Report R2001/0999 to the Australian Fisheries Management Authority.