Introduce an overall catch limit (quota cap or total allowable catch) by fishery or fleet

How is the evidence assessed?
  • Effectiveness
    not assessed
  • Certainty
    not assessed
  • Harms
    not assessed

Study locations

Key messages




  • Abundance (5 studies): Four before-and-after studies (two replicated) in the South Atlantic Ocean, the North Atlantic Ocean and the North Sea reported that following the introduction of overall catch limits for fish there was a higher abundance or biomass of hakes, Atlantic herring and Atlantic halibut, compared to before. One replicated, controlled study of fish stocks worldwide found that overfished stocks of tunas and billfishes had faster increases of biomass when managed using overall catch limits, compared to stocks with other types of control or no management.
  • Reproductive success (1 study): A global review reported that after overall catch limits and minimum landing size were introduced there was strong recruitment of broadbill swordfish for one key stock, while recruitment for four other stocks could not be assessed due to limited data.
  • Survival (2 studies): One before-and-after study and one replicated, controlled study in the North Atlantic Ocean and worldwide found that for fish species with overall catch limits there was a decrease or lower fishing mortality, compared either to before implementation or to stocks without catch limits and those with other controls.



  • Reduction of unwanted catch (1 study): One replicated, before-and-after study in the North Sea and North Atlantic Ocean found that overall catch limits did not reduce unwanted megrim catch despite a reduction in discards, however this was due to retention of more small but legal-sized megrim, previously discarded.
  • Reduction of fishing effort (1 study): One replicated, before-and-after study in the North Sea found that when annual total allowable catch limits were changed (increased or decreased), half of the otter trawl fleet had corresponding changes in fishing effort (increased or decreased), but there were no changes for the beam trawl fleet.
  • Stock status (1 study): One global systematic review found that in terms of reaching biomass-based management targets fisheries with fleet-wide catch quotas were no different to fisheries managed either by catch shares or effort controls. However, along with catch share fisheries, fewer catch quota stocks were over-exploited than those with effort controls.

About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

  1. A replicated, before-and-after study in 1965–1984 of four areas of seabed in the southeast Atlantic Ocean, off South Africa and Namibia (Andrew & Butterworth 1987) reported that introducing overall catch limits for Cape hake Merluccius capensis resulted in increased abundance following a period of decline due to overfishing. Data were not tested statistically. In the period after the introduction of catch limits in 1975, total catch rates of Cape hake between 1976–1984 steadily increased, from 0.6 to 1.0 (data reported as an index of catch/unit effort). In the years prior (1965–1975), when there were no catch limits, catch rates declined from a maximum of 2.5 in 1966 to 0.8 in 1975. In addition, for three of the four areas hake stock biomass was assessed to have attained levels required for maximum sustainable yield in the years when quotas were set. Catch limits for hake were introduced in 1975 for four management areas (International Commission for the Southeast Atlantic Fisheries Divisions) covering major hake trawling grounds extending along the coasts of South Africa and Namibia. Catch rate indices were derived from catch and effort statistics data collected for the four areas between 1965 and 1984.

    Study and other actions tested
  2. A review in 2000 of broadbill swordfish Xiphias gladius fisheries worldwide (Ward et al. 2000) reported that after catch limits (quotas) and minimum landing sizes were introduced for the species, strong recruitment was found for one stock, whilst four others could not be assessed due to lack of data. Data were not tested statistically. Authors reported that after measures were introduced in 1994, recruitment of age-1 swordfish in the North Atlantic area was ‘strong’ in 1997 and 1998 (no data reported), while in the four other areas there were either wide fluctuations in recruitment or there was no reliable assessment of the stocks (see paper for details). In addition, compliance was generally poor and between 19–37% of swordfish landed from various countries fishing in the Atlantic in 1998 were under the recommended size (125 cm). In 1994, total allowable catches and minimum size limits for swordfish were recommended by the International Commission for the Conservation of Atlantic Tunas. However, the recommendations were enforced by some, but not all member states, and following the limits some vessels relocated to other regions and an increase in discarded swordfish was reported. Five swordfish fisheries were reviewed to develop guidelines for the assessment and management of developing swordfish fisheries.

    Study and other actions tested
  3. A before-and-after study in 1990–2000 of an extensive area of seabed and mid-water in the South Atlantic Ocean off the coast of Namibia (van der Westhuizen 2001) reported that after introduction of annual catch limits (total allowable catches) the biomass of hake Merluccius spp. increased. Data were not tested statistically. During the period after new total allowable catches were set (at 60,000 t), total hake biomass increased from 500,000 t in 1990 to 900,000 t in 1992. Biomass gradually declined in the years following, to 550,000 t in 1997, but increased again between 1998–2000 (1,000,000–1,300,000 t). During the same period, total allowable catches increased to 87,000 t in 1992 and to 160,690 t in 1999, and numbers of vessels participating in the fishery rose from 55 (1991) to 105 (2000). The authors noted that the decline in hake biomass between 1992–1997 was probably the result of anomalous environmental conditions in 1993–1995. New, conservative catch limits were imposed from 1990 following Namibian Independence. From 1992–1997, catch limits were set as 20% of the fishable biomass. However, conflicting estimates of abundance – research survey vs commercial indices – lead in 1997 to the introduction of new methodology to calculate abundance (see paper for details). Biomass data was estimated from 18 research surveys between 1990–1999. Bottom trawl transects were done 20 miles apart to assess bottom-dwelling hake. Mid-water hake were assessed using acoustics.

    Study and other actions tested
  4. A before-and-after study in 1962–2003 of pelagic areas across the North Sea, northern Europe (Simmonds 2007) reported that management actions, including the introduction of overall catch limits (total allowable catches), resulted in an increased herring Clupea harengus spawning stock biomass following a major decline, compared to a similar period of decline caused by overfishing and little or no management. Data were not tested statistically. Herring spawning stock biomass (for which the target for sustainable catches was 800,000 t) increased between 1997–2003 after the implementation of total allowable catches, compared to the previous period 1987–1996 of declining biomass (after: 600,000–1,750,000 t; before: 500,000–1,250,000 t). Compared to the years 1970–1978, following a similar decline in biomass, total allowable catch (TAC) management resulted in higher levels of stock biomass than with no management (TAC: 600,000–1,750,000 t; no TAC: 50,000–450,000 t). Fisheries data and scientific advice for North Sea herring were compared for two, 17-year periods with similar trends (1962–1978 and 1987–2003) encompassing two years (1970 and 1995) with critically low levels of depleted spawning stock biomass and high fishing mortality. During the second period in 1997, an EU/Norway agreement on management actions (implementation of total allowable catches) was introduced. During the first period of excess fishing, no management action (limited scientific advice and failure to implement total allowable catches) and a rapidly declining stock biomass resulted in 1978 in total collapse and closure of the entire fishery.

    Study and other actions tested
  5. A replicated, before-and-after study in 1997–2004 of seabed across the whole of the North Sea, northern Europe (Reiss et al. 2010) found that changes in annual total allowable catch limits (TACs) for six main commercial fish species resulted in similar direction changes in fishing effort for half of the otter trawl fleets, but there were no changes in effort for beam trawl fleets. For otter trawl fleets, corresponding changes in fishing effort with changing TACs (i.e. decrease or increase in effort with decreasing or increasing TAC) were found between four of eight comparisons: cod Gadus morhua TACs and the English and Scottish, but not German trawl effort; whiting Merlangius merlangus TAC and English trawl effort; and saithe Pollachius virens TACs and Norwegian, but not German, trawl effort. No effect of changes was found for haddock Melanogrammus aeglefinus TACs and Scottish and Norwegian otter trawl effort. For beam trawl fleets, there were no effects of changes in plaice Pleuronectes platessa and common sole Solea solea TACs on trawl effort of five national fleets (data presented as graphical trends – see original paper). Fishing effort and total allowable catch data for 1997–2004 were compiled for the North Sea. Otter trawl and beam trawl effort targeting the main demersal (bottom) fish species were examined from the German, English, Norwegian, Dutch and Scottish fleets. These data were summarised to produce estimates of the number of hours of fishing effort/year for each International Council for the Exploration of the Sea (ICES) management rectangle (0.5° latitude by 1.0° longitude) and aggregated across rectangles to provide annual North Sea effort estimates. Annual total allowable catch data were used for the main demersal commercial target species.

    Study and other actions tested
  6. A systematic review in 2000–2004 of 11 marine regions worldwide (Melnychuk et al. 2012) found that fisheries that introduced fleet-wide catch limits (quota caps) did not have improved biomass-based performance targets compared to fisheries managed by catch shares (divisions of annual fleet-wide quotas among individuals or groups) or fishing effort controls but, along with catch share fisheries, had lower rates of over-exploitation than fishing effort controls. Across stocks, there was no difference in the ratios of fish stock biomass relative to the target management biomass between catch share and non-catch share fisheries (data presented as statistical results). In addition, there was no difference in the proportion of stocks subject to over-exploitation (measured as current exploitation rate to target exploitation rate) between stocks under catch shares (9%) and fleet-wide quota limits (13%). However, over‐exploitation was higher (41%) in stocks under effort controls (like days-at sea limits or size-based limits). Data for 2000–2004 were extracted from a global fisheries stock assessment database (see original paper for details) and compiled from other sources including stock assessment documents and fishery management plans. A global meta-analysis examining trends in biomass and exploitation rates was performed for 345 stocks of 158 species from 11 regions. See original paper for full details of assessment methods and metrics used.

    Study and other actions tested
  7. A replicated, before-and-after study in 2008–2012 of a fished area of seabed in the North Sea/North Atlantic Ocean, UK (Macdonald et al. 2014) found that an overall catch limit (total allowable catch) for common megrim Lepidorhombus whiffiagonis did not result in a reduction of unwanted catch, and even though it reduced discarding, this was due only to higher retention of small but legal sizes of fish previously discarded. Across all catches, the proportion of total megrim catch discarded decreased in years with higher quotas (2011–2012, average 20% in 2012) compared to years with lower quotas (2008–2010: data reported as statistical results, average 54% in 2009). However, the reduction was mainly the result of a lower proportion of fish categorized by the crew as ‘small’, and discarded, as quota increased (2012: 0.1, 2009: 0.39) even though >99% were above minimum landing size. The likelihood of a fish being discarded decreased with increasing quota (data reported as statistical result) indicating catch limits were only regulating the amount of megrim landed but had no effect on fishing mortality, particularly of smaller sizes. Megrim total allowable catch was reduced in the North Sea between 2004–2007 in response to low biomass. However, increasing megrim biomass and low catch limits led to high discarding and limits were increased again between 2010–2013. Data were collected by on board sampling of catches on eight demersal (bottom) fishing vessels based in the Shetland Islands, Scotland, and working in the mixed demersal fishery in the northern North Sea/North Atlantic Ocean (International Council for the Exploration of the Sea (ICES) Division IVa). Observer trips (2–8 d) were undertaken between May 2008 and March 2012. A total of 25 trips (22 twin trawl and 3 seine) and 407 hauls were sampled. Megrim length was recorded from the retained and discarded portions of the catch. Discards were categorized as “small” or “bruised”.

    Study and other actions tested
  8. A replicated, controlled study in 2004–2014 of marine areas worldwide (Pons et al. 2016) found that previously depleted stocks of tunas Scombridae and billfishes (swordfish Xiphiidae, marlins and sailfish Istiophoridae) managed under catch limits (total allowable catches) resulted in faster increases of biomass and lower fishing mortality compared to stocks with at least one type of other management control or no management measures. For both tuna and billfish stocks, annual average rates of change of biomass were higher and increasing, and rates of fishing mortality were higher and decreasing, for stocks currently managed under total allowable catch quotas (8 stocks) than stocks with some management measures including minimum size regulations or seasonal closures (28 stocks) or no measures (12 stocks: data presented graphically). In addition, the number of years since quota implementation was the most important for increasing biomass compared to the other control types, but measures such as minimum size regulations and seasonal closures were more important in reducing fishing mortality (see paper for data). Data were obtained from stock assessment outputs compiled from a global stock assessment database (see original paper for details); for 40 stocks of 13 species, seven species of major commercial tunas and six species of billfishes from at least 48 stocks defined globally. Data for management variables were compiled from information available on the websites and reports of different management organisations and through personal communication with their staff.

    Study and other actions tested
  9. A before-and-after study in 1970–2010 of an extensive area of seabed in the Northwest Atlantic Ocean, Canada (Trzcinski & Bowen 2016) reported that management actions that included reductions in catch limits (total allowable catch), resulted in a decrease in fishing mortality and an increase in biomass of Atlantic halibut Hippoglossus hippoglossus over a period of 10 years, compared to the previous period. Data were not tested statistically. During the period of management activity from 1988–2009, both a steep increase in fishing mortality and a sharp decline in halibut spawning stock biomass since 1984 were halted: fishing mortality decreased to below targeted levels for sustainable harvest, while biomass gradually increased to well above target limits for sustainable harvest (data reported as trends in the ratios of fishing mortality and biomass to the target sustainable fishing mortality and biomass). In 1988, the first halibut total allowable catch limit was set (3,200 t) for fishing on the Scotian Shelf and southern Grand Banks and reduced in small steps to 850 t in 1994. A minimum size limit (81 cm) was introduced in 1988, fully implemented by 1994. Following signs of population recovery, the total allowable catch was increased again in small steps beginning 1999. Halibut size and age data were obtained from commercial catches and catch-rate data from groundfish and halibut surveys. The data were used in a stock assessment model that estimated population dynamics and exploitation rates over the period 1970–2010.

    Study and other actions tested
Please cite as:

Taylor, N., Clarke, L.J., Alliji, K., Barrett, C., McIntyre, R., Smith, R.K., and Sutherland, W.J. (2021) Marine Fish Conservation: Global Evidence for the Effects of Selected Interventions. Synopses of Conservation Evidence Series. University of Cambridge, Cambridge, UK.

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