Management options

Management options in mmsyFM

This page explains the management options of the app in a real-world context: what each option means, how the model represents it, where it is used in practice, and what to watch for. It applies to any system, whether the Atlantic example, the Gulf of Thailand draft or your own tables.

Mixed fisheries are hard to manage because one fleet, and often one day at sea, catches many species at once. Every measure below is a different answer to the same question: how much fishing, by whom and where, keeps every stock safe while delivering the most food, income or jobs?

1. Reference points: what “safe” and “good” mean

In practice. ICES and GFCM advice sets the trigger of harvest control rules at Bpa (often called MSY Btrigger) and requires that the risk of falling below Blim is at most 5%. FAO reports stocks below 0.8 Bmsy as overfished. The defaults of the app follow these conventions; all can be changed.

2. Objectives: what to maximise

Whatever the objective, the safeguard is applied: no stock may fall below Bpa at equilibrium. The stock that limits the solution is the choke stock. In mixed fisheries this is often a slow-growing or highly catchable species (e.g. reef fish, sharks, rays), and protecting it costs yield of the healthier stocks caught alongside it.

Single-stock MSYs do not add up. Because stocks are caught together, fishing every stock at its own Fmsy is not possible; the multispecies MSY is lower than the sum of single-stock MSYs.

3. Effort limits (input control)

Most tropical and many mixed fisheries are managed by limiting fishing effort (licences, vessel numbers, days at sea, gear) rather than catches, because catches are hard to monitor across many species and landing sites.

• One global cap. A single limit on total days, shared by the fleet segments in their current proportions. Simple, but it cuts every fleet by the same proportion, including fleets that do not catch the choke stock.
• Caps by segment. Separate limits for e.g. trawl, purse seine and small-scale fisheries (licences or days by fleet). Cuts can target the fleet that drives the choke stock, so they usually give a higher yield for the same safeguard.
• Viability floor. With caps by segment, no segment is cut below a minimum share of the days. This reflects social and political constraints: livelihoods of small-scale fishers, food security, or keeping a fleet viable. It is an explicit trade-off between yield and equity.
• Capacity (set-up tab) is the registered fishing capacity: the most days a fleet could fish. Without limits, the days actually fished are those of the vessels that make a profit (open access).

What the model adds. Fishers respond to limits: when a segment’s days are capped, its vessels still choose where to fish by expected profit, and unprofitable vessels stay in port. An effort limit therefore does not reduce fishing mortality equally on all stocks; effort can concentrate on the most valuable grounds, which may be those of the choke stock.

4. Static and feedback management

• No control (open access). The fleet at its capacity, limited only by profitability. The baseline to compare against.
• Fixed MMSY effort. Each segment fixed at the days of the safeguarded optimum (tab 4): a plan set once, like licence caps that are not revised. It works at equilibrium, but does not respond if stocks decline because of bad years, assessment errors or poor compliance.
• Harvest control rule (HCR). Effort is revised every year from the assessed status of the stocks: full effort when the stocks are above the trigger, reduced effort below it, down to a minimum at Blim. This is feedback (adaptive) management, as used by ICES, GFCM and tuna RFMOs, here applied to effort.

5. What is assessed

Assessments are expensive, and in most multispecies fisheries only some stocks are assessed. The rule can only respond to what is assessed.

• All stocks (data-rich): every stock has its own assessment.
• Indicator species: a few well-monitored stocks stand for the fishery (e.g. one demersal, one pelagic, one invertebrate). Cheap, but stocks that are not assessed can decline unnoticed.
• Species complexes: stocks assessed together as a group (e.g. “reef fish”, “demersal fish”), from pooled catch and effort or pooled survey biomass. Common where species are landed and recorded together. A healthy dominant species can hide a depleted member of the group.
• Nominal assignment: each stock is managed through one fleet (e.g. a pelagic stock “belongs” to the purse seine), although other fleets catch it. Simple to administer, but the other fleets are not constrained and the burden falls on the assigned fleet.

In the app (tab 5), tick the rules “HCR, indicator species”, “HCR, species complexes” or “HCR, nominal assignment” next to the “HCR, value-weighted” rule (all stocks assessed) to see what each assessment strategy costs or risks, with the same random draws. Defaults come from the system and can be edited: one indicator per fleet segment (the stock of highest value to it), the species groups as complexes (pooled biomass or pooled CPUE), and each stock assigned to the segment for whose fishery it matters most. The differences show most with the weakest link, where a complex or a missing assessment can hide the depleted stock that would otherwise cut effort. In the package: hcr_effort(assessed =, complexes =, assign =) and ms_assessment_presets().

6. Closures

Closures act on where and when fishing happens, not on how much.

• Metier closures (fixed): close one or more metiers (a gear on a ground) all year or for some months, fully or for part of the ground. Real-world examples: closed seasons during spawning (e.g. the seasonal closures for Indo-Pacific mackerel in the Gulf of Thailand), trawl bans in the coastal zone reserved for small-scale fisheries, reef or nursery area closures and marine protected areas. Often more practical than species rules, because they are easy to monitor and enforce.
• Closure triggered by a stock: the metiers that drive one choke stock are closed in proportion to how depleted it is (fully at Blim, reopened at the closure trigger). This protects the choke stock without cutting whole fleets, but needs an assessment of that stock.

Displacement. Closed days do not disappear: vessels move to the open metiers by expected profit. A closure can therefore increase pressure on other stocks; the model shows this directly.

7. Uncertainty: why rules are tested in simulation

A rule that looks good at equilibrium can fail under these uncertainties. Every rule is therefore run many times with the same random draws, so differences between rules are due to the rules, not to chance.

8. Reading the results

• Risk: P(B < Blim) for each stock and for any stock; a common standard is at most 5%.
• Status: B/Bmsy and P(B > 0.8 Bmsy) (the FAO “not overfished” criterion).
• Benefits: catch, revenue, profit.
• Stability: average annual variation of catch and of effort.
• Employment and equity: days fished by fleet segment.

No rule is best on everything. The decision table and trade-off plots make the choice explicit: how much catch or profit is given up for how much less risk, and which fleets bear the cost.