Unexploited Stock Research Articles

Age, growth, and biomass projections of red porgy Pagrus pagrus (Teleostei, Sparidae) after the fishery collapse in southern Brazil

AbstractA formerly unexploited stock of red porgy that was intensely fished along southern Brazil in the 1970s collapsed in less than a decade. Subsequently, population dynamics or stock status has not been reported, so we updated growth parameters by analysis of otolith annual‐increments and micro‐increments. Growth did not change after the stock collapsed, which suggested no density‐dependent effect. Given the lack of growth changes and data‐limitation, we used an age‐structured assessment model set until 1984 to forecast relative spawning biomass and assess recovery possibilities. Projections showed that a 19 years moratorium would have been necessary for the stock to rebuild. The model suggested an annual catch of at least 170 t in the post‐collapse period because the stock did not recover, and a small‐scale hand‐line fishery with unreported landings continued fishing. Our results reinforced the need for more curated and up‐to‐date data collection to properly assess and manage this formerly abundant stock.

The carrying capacity of the seas and oceans for future sustainable food production: Current scientific knowledge gaps

AbstractThe expected increase in global food demand, as a consequence of a rising and wealthier world population, and an awareness of the limits and drawbacks of modern agriculture, has resulted in a growing attention to the potential of the seas and oceans to produce more food. The capture production of presently exploited marine fish stocks and other species has more or less reached its maximum and can only be slightly improved by better management. This leaves four alternative options open to increase marine food production: (1) manipulating the entire food web structure via removal of high trophic level species to allow an increasing exploitation of low trophic level species, (2) harvesting so far unexploited stocks, such as various fish species from the mesopelagic zone of the ocean or the larger zooplankton species from polar regions, (3) low‐trophic mariculture of seaweeds and herbivorous animals, and (4) restoration of impoverished coastal ecosystems or artificially increasing productivity by ecological engineering. In this paper, we discuss these four options and pay attention to missing scientific knowledge needed to assess their sustainability. To assess sustainability, it is a prerequisite to establish robust definitions and assessments of the biological carrying capacity of the systems, but it is also necessary to evaluate broader socio‐economic and governance sustainability.

How does climate change affect a fishable resource? The case of the royal sea cucumber (Parastichopus regalis) in the central Mediterranean Sea

Holothurians or sea cucumbers are key organisms in marine ecosystems that, by ingesting large quantities of sediments, provide important ecosystem services. Among them, Parastichopus regalis (Cuvier, 1817) is one of the living sea cucumbers in the Mediterranean actively fished for human consumption mainly in Spain, where it is considered a gastronomic delicacy. In the Strait of Sicily (central Mediterranean Sea), this species is not exploited for commercial use even if it is used as bait by longline fishery. P. regalis is frequently caught by bottom trawling and discarded at sea by fishers after catch, and because of its capacity to resist air exposition (at least in cold months), it is reasonable to consider that it is not affected by fishing mortality. Having observed a significant decrease in abundance since 2018, the possible effects of some ecological factors related to current climate change (i.e., temperature and pH) were sought. Generalized additive models (GAMs) were applied to investigate the relationship among the abundance of P. regalis and environmental variables and fishing effort. Long time series of P. regalis densities (2008–2021) were extracted from the MEDITS bottom trawling survey and modeled as function of environmental parameters (i.e., salinity, dissolved oxygen, ammonium, pH, and chlorophyll α) and fishing effort (i.e., total number of fishing days per gross tonnage). Our results showed that this species prefers the soft bottoms (50–200 m) of the Adventure Bank and Malta Plateau, and its distribution changed over time with a slight deepening and a rarefaction of spatial distribution starting from 2011 and 2017, respectively. In addition, a positive relationship with pH concentration in surface waters during the larval dispersal phase (3-year lag before the survey) and nutrient concentration at sea bottom (1-year lag) has been found, suggesting that this species is sensitive to climate change and food availability. This study adds new knowledge about the population dynamics of an unexploited stock of P. regalis under fishing impact and environmental under climate change in fisheries management.

THEORETICAL PROPOSAL OF NEW MOLECULES TO REPLACE CHLORDANE

Chlordane was a most used as insecticide by farmers. Today, it is banned owing to its toxicity, bio-accumulative and persistence properties. Since then ¶tits prohibition has created more unexploited stocks in the environment. ¶In order to still allow the use of these products, we theoretically proposed new molecules derived from it. To meet this objective, we have reduced the number of chlorine atoms in the chlordane molecule to four. Thus, t¶he QSAR and QSPR models already established made it possible to theoretically determine and the of 70 molecules proposed. ¶The analysis of the results allowed to retain two molecules. These are (1S,2S,3R,6R,7R)-3,8,10,10-tetrachlorotricyclo[5,2,1.02,6]dec-8-ene) and (1R,2R,3R,6R,7S)-3,8,7,8-tetrachloro-tricyclo[5.2.1.02,6]dec-8-ene. Both¶bothbbbbbbbggddaaa molecules seem to present the best values of and BCF.¶¶In order to also verify the influence of fluorine on chlordane, we carried out another study. In this part, we have considered the two abovementioned molecules on which a substitution of four hydrogen atoms by four fluorine atoms was made. This gave the following two molecules: (1R,2S,3S,4R,6R,7S)-3,8,10,10-tetrachloro-1,4,7,9-tetrafluorotricyclo[5,2,1,02,6]dec-8-ene and (1S,2S,3R,4S,6S,7S)-1,3,7,8-tetrachloro-4,9,10,10-tetrafluorotricyclo[5,2,1,02,6]dec-8-ene. Both ¶both molecules show higher BCF values ​​than the formers. This would mean that organochlorine compounds which have fluorine atoms are storable. Besides, when we consider the fact that these molecules are more absorbent, (1R,2S,3S,4R,6R,7S)-3,8,10,10-tetrachloro-1,4,7,9-tetrafluorotricyclo[5,2,1,02,6]dec-8-ene and (1S,2S,3R,4S,6S,7S)-1,3,7,8-tetrachloro-4,9,10,10-tetrafluorotricyclo[5,2,1,02,6]dec-8-ene appear to be less accumulable and less toxic than (1S,2S,3R,6R,7R)-3,8,10,10-tetrachlorotricyclo[5,2,1.02,6]dec-8-eneand (1R,2R,3R,6R,7S)-3,8,7,8-tetrachloro-tricyclo[5.2.1.02,6]dec-8-ene.

Growth, mortality, exploitation rate and recruitment pattern of Octopus cyanea (Mollusca: Cephalopoda) in the WIO region: A case study from the Mafia Archipelago, Tanzania

Octopus cyanea is a commercially important cephalopod in the Western Indian Ocean (WIO) region, but scientific information to inform management strategies for the species is limited. A study was conducted in 2014, 2015, 2017 and 2018 to investigate biological parameters including growth, mortality, exploitation rates and recruitment patterns in the sea around Mafia Archipelago, Tanzania. Virtual population analysis (VPA) indicated differential mortality between two sampling sites; the lowest and highest fishing mortality of F = 1.5yr-1 and F = 2.7yr-1 were observed in Bwejuu (Dorsal mantle length, DMT = 18-20 cm) and Jibondo (DMT = 8-12 cm) fishing villages, respectively. The maximum exploitation rate (Emax), which gives the maximum relative yield per recruit, was estimated at 0.380 and 0.379 for Jibondo and Bwejuu, respectively. The exploitation rates E 0.5, which corresponded to 50% of the unexploited stock relative biomass per recruit, were estimated at 0.248 for Jibondo and 0.247 for Bwejuu. These values differ greatly from the exploitation rates of 0.53 and 0.41 for Jibondo and Bwejuu, respectively, suggesting that the stock of O. cyanea is probably being overfished both in terms of yield per recruit and biomass per recruit. The stock-recruit pattern was observed to be continuous year-round, with the peak being between May and July. Since the peak in recruitment of both areas coincides with the south-east monsoon (SE Monsoon) and the level of maximum sustainable yield has been overshot, it is recommended that management plans are implemented that will reduce effort while increasing biomass, for example, implementing temporal octopus fishery closures at a village level.

State entrepreneurship in New South Wales’ trawl fishery, 1914-1923

In 1914, the New South Wales (NSW) Government decided to alter its fisheries policy, with the development of an offshore trawling industry supplanting support for inshore fishing as its key development objective. Accordingly, between 1915 and 1923 the NSW Government operated a commercial trawling industry designed to fish previously unexploited fish stocks on the state’s continental shelf. The State Trawling Industry (STI) was designed to meet a mix of social and economic policy goals, with the NSW Government controlling all parts of the production line from catching to selling produce. This article examines the business structure of the enterprise to reveal the reasons for its economic failure. It argues that government entrepreneurship created a new consumer market and unintentionally paved the way for the rise of a modern private trawling industry.

Mortality, exploitation rate and recruitment pattern of African lungfish, <i>Protopterns annectens</i> (Owen, 1839) from Orashi and Sombreiro Rivers, Niger Delta, Nigeria

The study determined mortality, recruitment and exploitation rate of a commercially important fish, Protopterus annectens from Orashi River and River Sombriero between October 2013 and September 2014. A total of 347 individuals of P. annectens were collected fortnightly by commissioned fishers using multiple gear. The total mortality was 3.225 yr-1, natural mortality was 1.465 yr-1, fishing mortality was 3.107 yr-1 and exploration rate was estimated as 0.355. The exploration rate (Emax) that gave maximum relative yield per recruit was 0.355. The exploitation rate at which marginal increase was E= 10% was 0.263. The exploitation rate E=50% which corresponded to 50% of the margin or unexploited stock was 0.229. The mean ratio of length at first capture (LC) and asymptotic length (L∞) was 0.050 while that of natural mortality (M) and growth rate (Kyr-1) was 2.5. The recruitment pattern showed all year round recruitment with peak and latent periods. The parameters obtained were L∞ = 82.95, K = 0.18yr-1, T = 29.10C, L mean = 29.5cm, Lmax = 74.97cm. The fish was available all year round. There is need to ensure sustainability of the species in rivers by checking activities of indigenous inhabitants whose commercial activities may destroy the favourable environment of the fish.Keywords: Mortality, exploitation rate, recruitment, Protopterus annectens

The Recruitment Pattern of Liza falcipinnis from Elechi Creek, Upper Bonny, Niger Delta, Nigeria

The Recruitment Pattern of Liza falcipinnis from Elechi creek of Upper Bonny, Niger Delta, Nigeria was studied from (March 2009-January 2010). The pattern showed all year round recruitment with two peaks (one major and one minor) during the period of the study. The parameters obtained were L∞ = 19.96, K = 0.40 y﹣1, C = 0, WP = 0, to = 0. The C indicates the amplitude of seasonal growth oscillations (that is, the magnitude of the growth patterns) and has values ranging from 0 to 1.0. Winter Point (WP) indicates the times of the year during which growth is minimal. The percentage recruitment for the different months were: March 2009 (15.0%); April (23.40%); May (14.0%); June (11.3%); July (10.0%); August (7.30%); September (1.8%); October (0.5%); November (2.2%); December (8.0%) and January 2010 (6.0%). The exploitation rate (Emax) that gives maximum relative yield-per-recruit was 0.424. The exploitation rate at which marginal increase occurred in the relative yield-per-yield was 10% of its value at E = 0, whereas (E0.1) was observed to be 0.357. The exploitation rate (E0.5) which corresponds to 50% of the virgin (that is, the unexploited stock) relative biomass-per-recruit was estimated to be 0.279. The mean ratio of length-at-first capture (L∞) and asymptotic length (L∞) was 0.060, while that of natural mortality (yr﹣1) and growth rate (yr﹣1) was 1.00. Yield per increased gradually with increase in exploitation and Biomass per recruit declined with increase in exploitation. An all year round recruitment, with one high pulse arid a low pulse was established for the species. Research should be carried out to ascertain the influence of environmental factors on the growth and recruitment of the species.

Estimating fisheries reference points from catch and resilience

AbstractThis study presents a Monte Carlo method (CMSY) for estimating fisheries reference points from catch, resilience and qualitative stock status information on data‐limited stocks. It also presents a Bayesian state‐space implementation of the Schaefer production model (BSM), fitted to catch and biomass or catch‐per‐unit‐of‐effort (CPUE) data. Special emphasis was given to derive informative priors for productivity, unexploited stock size, catchability and biomass from population dynamics theory. Both models gave good predictions of the maximum intrinsic rate of population increase r, unexploited stock size k and maximum sustainable yield MSY when validated against simulated data with known parameter values. CMSY provided, in addition, reasonable predictions of relative biomass and exploitation rate. Both models were evaluated against 128 real stocks, where estimates of biomass were available from full stock assessments. BSM estimates of r, k and MSY were used as benchmarks for the respective CMSY estimates and were not significantly different in 76% of the stocks. A similar test against 28 data‐limited stocks, where CPUE instead of biomass was available, showed that BSM and CMSY estimates of r, k and MSY were not significantly different in 89% of the stocks. Both CMSY and BSM combine the production model with a simple stock–recruitment model, accounting for reduced recruitment at severely depleted stock sizes.

Long-term changes in the age structure, mortality and biomass of the king weakfish Macrodon atricauda (Günther, 1880) in southern Brazil: Is it resilient enough to avoid collapse?

Worldwide overfishing has caused the collapse of populations of excessively exploited marine fish. The coastal demersal sciaenid fish “pescadinha”, Macrodon atricauda, has been intensely fished since the 1960s along the southern Brazilian coast, resulting not only in decreasing abundance but also in increasing growth rates and decreasing age and length at first maturity. We analyzed the time series of several population indicators, such as catch per unit effort (CPUE), age composition of the stock, total and natural mortality and exploitation rate to investigate the long-term impact of fishing on the mortality, age structure and biomass of M. atricauda. Furthermore, we modeled the time-trajectory of the total biomass under an assumption of constant recruitment and discuss the limits of resilience and the risk of collapse of the fishery. The CPUE (kg/day at sea) has decreased almost 50%. Total mortality increased from 0.5yr−1 in the almost unexploited stock in the 1950s to around 1.9yr−1 in the 2000s, while the age structure changed dramatically: fishes in the landings were not older than 9 years old in the 1960s, 7 years old in the 1970s and no fish over 5 years old has been observed since the 1990s. Taking into account the growth changes, a deterministic model estimated a reduction of 67% in total biomass over five decades. In the last 30 years, the stock has withstood high exploitation rates (between 0.5 and 0.7yr−1) and has suffered a steady decline in biomass but has not collapsed, probably due to life-history traits that favor resilience: early maturation and rapid individual growth. However, the stock is at risk of collapse, taking into consideration the present non-stabilized level of high mortality, changed age structure and life history, low biomass and high exploitation rates. A precautionary approach suggests the need to reduce fishing effort.

Modeling sardine and anchovy low-frequency variability

The group of small pelagics (sardines, anchovies, herrings, etc.) is the largest source of wild marine protein worldwide, providing annually between 21% and 43% of the total marine capture production during at least the last 60 y (1). Even when large-scale fisheries have been operating for decades to centuries, their assessment and management remains difficult and highly uncertain (2). Such challenges are mainly because these species do not conform to the traditional assumptions of population dynamic models, such as a steady level of unexploited stock (carrying capacity), or that the influence of climate on the population is negligible when compared to fishing mortality. In other words, it has been clear for decades that the traditional toolbox of fisheries management is clearly unsuitable to these massive resources. The paper by Lindegren et al. (3) in PNAS provides an alternative framework to study and ultimately to develop management-supporting knowledge for these types of resources.

Sexual dimorphism in size, age, maturation, and growth characteristics of boarfish (Capros aper) in the Northeast Atlantic

Abstract Hüssy, K., Coad, J. O., Farrell, E. D., Clausen, L. W., and Clarke, M. W. 2012. Sexual dimorphism in size, age, maturation, and growth characteristics of boarfish (Capros aper) in the Northeast Atlantic – ICES Journal of Marine Science, 69: 1729–1735. Boarfish (Capros aper) have, in recent years, become of increasing commercial importance due to their apparent increase in abundance in the Northeast Atlantic. This study presents detailed biological information relevant to understanding stock structure and dynamics. Boarfish are a long-lived species that reach a maximum age of >30 years. The size distribution is skewed towards larger sizes, as expected from an unexploited stock with sexual dimorphism, where females are, on average, larger than males. No seasonal effects occur in size distribution and sex ratio, indicating that females and males stay together in shoals throughout the year. Females become increasingly dominant in abundance at larger sizes and older ages. There is no significant difference in length at maturity nor age at maturity between the two sexes, with L50 = 9.7 cm and A50 = 3.4 years. Growth is dimorphic following von Bertalanffy growth characteristics, with a common t0 = –2.897, but significantly different K and Linf values, where females have K = 0.145 and Linf = 16.519 and males have K = 0.181 and Linf = 14.437. The sexual dimorphism in size is thus caused by sex-specific growth and longevity patterns.

Growth discontinuity in males of the deep‐water giant red shrimpAristaeomorpha foliaceain the Mediterranean Sea

AbstractThe deep‐water giant red shrimp,Aristaeomorpha foliacea(Risso, 1827) (Decapoda, Aristeidae), represents a highly valuable resource for bottom trawling in the Mediterranean Sea. Recent assessments have described both a worsening of the status of the traditionally exploited stocks and low levels of annual‐based instantaneous natural mortality (M 0.4–0.7 year−1) in the unexploited stocks. The mortality (M) figures, however, are in contrast with the longevity (Tmax) of 3–4‐year males and 4–5‐year females, estimated from classical length frequency distribution analysis. Reduced growth (after the onset of sexual maturity) and pile‐up of older individuals in the larger size classes have been considered to be reasonable explanations for the contrast between M and Tmax. We propose that a discontinuity of the growth models could address this contrast. Because a clear discontinuity in sexual maturity is evident only in males, length frequency distribution data for different sets of males collected from the South of Sicily and the Maltese Islands deep bottoms (400–800 m) were fitted with both the classic (c) and double‐phased (d) von Bertalanffy growth formula (VBGF). According to the dVBGF, adult males sharply reduce their growth rate after reaching an age between 1.2 and 1.5 year, which corresponds to the estimates of age at sexual maturity (tmbetween 1 and 1.5 years). The reduction in growth determines a higher Tmax(7.3–9.5 year) and lower M (0.4–0.6 year−1) than previously derived on the basis of cVBGF estimates (Tmax3–4 years and M 1.2 year−1). The dVBGF results suggest that sexual dimorphism and length–sex segregation in giant red shrimps reflect an alternative life history strategy that is based mainly on growth reduction in adult males and it might be adopted for implementing more conservative assessments.

Stock and recruitment in North Sea herring ( Clupea harengus); compensation and depensation in the population dynamics

The recovery of a stock after severe exploitation is of major interest to fish ecologists and managers alike. Understanding the dynamics of recruitment at low stock sizes is crucial to the simulation of stocks as they recover. Compensation in recruitment has occurred in North Sea herring, and it was stronger after the collapse of the stock. The compensation appears to be a product of both increased production of larvae per spawner and increased survival to the juvenile stage. There is only slight evidence for depensation and the point at which North Sea herring has zero recruitment appears close to the origin. There is more variability in recruits per unit spawning stock size when the stock is smaller, this is probably as a result of the potential larger diversity in contributions from spawning components in an unexploited stock compared to an overexploited stock. Mimicking this dynamic in population models will increase the uncertainties in a projection of stock recovery. The lack of observations at higher stock sizes hinders our ability to compare the dynamics of recruit to SSB across the full range of stock sizes, however, recent apparent over-compensation at higher spawning biomass has resulted in less than one recruit per mature adult being produced per year.

The biology of the giant red shrimp ( Aristaeomorpha foliacea) at an unexploited fishing ground in the Greek Ionian Sea

Experimental trawl sampling was carried out along the Greek coast of the Ionian Sea between January and December 1997 at three depth zones: >500, 500–600 and <600 m, to study the distribution and population structure of an unexploited stock of the giant red shrimp ( Aristaeomorpha foliacea). The overall length–frequency distribution ranged between 16 and 62 mm (carapace length, CL), while the most abundant classes were between 40 and 48 mm CL. Length–frequency distribution was significantly dependent on depth. Medium-sized adults appeared in the shallower zone (>500 m), the young-of-the-year in the middle (500–600 m) and larger adults in the deepest (<600 m). The highest abundance was observed in the 500–600 m depth zone. Growth analysis indicated that the population was composed of two to three annual cohorts. The VBGF parameters were CL ∞=47.2 mm, K=0.564 per year, t 0=−0.13 years and CL ∞=64.086 mm, K=0.460 per year, t 0=−0.192 years for males and females, respectively. The asymptotic weight estimated from length–weight relationships was 32.6 and 67.3 g for males and females, respectively. The reproductive period, estimated from the maturity stages of females, is from summer to early autumn with a peak from June to August. The smallest mature female was 37.5 mm CL. Sex ratio was significantly higher than 1:1 in favor of males and no correlation with depth was found. Natural mortality was estimated at 0.57–0.66 and 0.47–0.55 per year for males and females, respectively. Total mortality was 0.58–0.62 and 0.53–0.62 per year for males and females, respectively. The abundance and the population dynamics of this species suggest that the population of the giant red shrimp in the Ionian Sea could provide a new target species to the deep-trawl fishery of the area.

Population dynamics of the common shrimp, Crangon crangon (L.), in Port Erin Bay, Isle of Man, Irish Sea

The population structure, growth, mortality, and size at sexual maturity of the common shrimp (Crangon crangon) were examined in Port Erin Bay, Isle of Man, Irish Sea between April 1995 and July 1998. For estimation of parameters of growth and mortality, monthly length–frequency data were analysed by ELEFAN. The population consisted mostly of the first year class, with a similar size composition each year. Regressions of body wet weight on carapace length indicated isometric growth (exponents very close to 3.0). Parameters of growth were estimated, using the modified von Bertalanffy growth function (VBGF) model incorporating seasonal variation in growth. Females grew faster and reached a larger size at age than males (K=1.09 yr −1 and L∞=18.5 mm CL for females, and K=0.90 yr −1 and L∞=15.1 mm CL for males). The maximum life span is estimated as 3.3 years. The recruitment pattern shows one major recruitment event per year. Total mortality (Z) by length-converted catch curve was estimated at 3.96 yr −1, fishing mortality (F) 0.36 yr −1, and natural mortality (M) 3.60 yr −1. The size at 50% sexual maturity for females ranged from 12.0 to 12.6 mm CL. Fecundity per recruit was 6.2×105. The exploitation rate (E) corresponding to 50% of the unexploited stock was 0.28 at L25, 0.32 at L50, and 0.35 at L75.

Some characteristics of a pike Esox lucius L. population in an Irish reservoir

A study of a stocked upland reservoir trout fishery, using gill nets, revealed the presence of a large stock of pike and poor trout numbers. The overall growth rate of pike was slow. Two year-classes of pike dominated, but the broad age and length frequency ranges, and the high average weight of individuals were indicative of an unexploited stock. Higher than normal water levels during the spawning period and into the summer period in both 1985 and 1986, coincided with the dominant pike year-classes recorded several years later.

Do Artificial Reefs Increase Regional Fish Production? A Review of Existing Data

We reviewed the scientific literature to determine whether the construction of artificial reefs increases the regional production of marine fishes. An evaluation of this technique is warranted by its high cost and logistical difficulty. Our review indicated that reef construction may have potentially deleterious effects on reef fish populations, including (1) increasing fishing effort and catch rates, (2) boosting the potential for overexploitation of stocks by increasing access to previously unexploited stock segments, and (3) increasing the probability of overexploitation by concentrating previously exploited segments of the stock. In contrast, the literature contained few studies that unambiguously demonstrated that artificial reefs increased regional fish production rather than merely concentrated available biomass. In addition, the literature on population regulation in reef fishes did not provide convincing evidence that reef fishes were limited by insufficient quantities of hard-bottom habitat. Consequently, potential positive and negative aspects of reef construction should be carefully evaluated prior to the addition of new reefs to marine environments.

Putting fisheries management back in places

Except, miraculously, for Jules Verne’s, scientific predictions always turn out to be wrong. However, the 3rd Millennium is coming, fisheries resources are going, and it is impossible to resist the urge to take the plunge and make a few predictions about the future of fisheries management, and of the scientific discipline. And no, the ‘s’ at the end of the title is not out of place: I shall suggest that in the future, fisheries management and its associated science will have to deal with ‘places’ far more than they have in the recent past. Indeed, I shall suggest that they will have to return, in many cases, to ancient modes of allocating fisheries resources to local communities, rooted in physical places. The trend now is going somewhere else, toward privatization of fisheries resources through Individual Transferable Quotas (ITQs) and similar instruments (Pauly, 1996), and there are also attempts to privatize the research scientists and the detailed assessment work that these instruments require (see e.g. Annala, 1996, and other contributions in Munro and Pitcher, 1996). However, this trend will crest when it is realized that, while eminently compatible with the acquisitive mood of our times, selfinterested exploitation schemes do not resolve, any more than the open-access schemes they might replace, the basic discrepancy between human and natural time scales. Many fisheries resource species, e.g. demersal fish in temperate waters and large predators on coral reefs, are long-lived, with natural mortalities of 0.1–0.2 yeary1, and often less (Pauly, 1980). This implies that, for exploitation to be sustainable, fishing must not extract more than about 10% of the stock biomass per year, especially in datasparse situations (Walters and Pearse, 1996). Even such low level of fishing mortality is sufficient, however, to quickly remove accumulations of large, old females – the source of most of the eggs and subsequent recruitment to stocks of long-lived fishes. This is so because the relationship between fish size and egg production is highly non-linear, with large females being far more fecund than an equivalent weight of small ones. Indeed, this non-linearity is so pronounced that for example one single ripe female red snapper, Lutjanus campecheanus, of 61 cm and 12.5 kg, contains the same number of eggs (9 300 000) as 212 females of 42 cm and 1.1 kg each (Bohnsack, 1990). The massive reduction of egg production, relative to unexploited stocks, coincident with the removal of such females, that occurs even when a very low fishing mortality is applied, is one of the reasons why exploited stocks fluctuate as much as they do, Reviews in Fish Biology and Fisheries 7, 125–127 (1997)

Effects of five years of intensive trawling on a previously unexploited smelt stock

Effects of five years of intensive trawling on a previously unexploited smelt stock