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Fishing-effort response dynamics in fisheries for short-lived invertebrates
Ocean and Coastal Management 165 (2018) 33–38
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Fishing-effort response dynamics in fisheries for short-lived invertebrates a,∗
a
a
a
a
A. Ben-Hasan , C. Walters , R. Louton , V. Christensen , U.R. Sumaila , H. Al-Foudari
T
b
a
Institute for the Oceans and Fisheries, University of British Columbia, Vancouver, BC, V6T 1Z4, Canada Ecosystem-based Management of Marine Resources, Environment and Life Sciences Research Center, Kuwait Institute for Scientific Research, PO Box 24885, 13109, Safat, Kuwait b
A R T I C LE I N FO
A B S T R A C T
Keywords: Fishing-effort responses Interannual bionomic equilibrium Invertebrate fisheries
In complex dynamic systems like fisheries, recognizing fishing-effort responses is as critical as understanding the biology of the exploited species for making sensible management decisions. In highly seasonal fisheries, it is theoretically possible for an “interannual bionomic equilibrium” to develop under open-access, where fleet dynamics may result in balanced year-to-year harvesting due to decreasing income per time fishing as biomass declines, without endangering the sustainability of the stock. However, in some conditions, this interannual bionomic equilibrium can be pathologically low leading to overfishing and amplification of extinction risks. Here we draw three cases from short-lived and fast-growing invertebrate fisheries to illustrate two distinct effort response dynamics: (a) fishing-effort responses that lead to healthy interannual bionomic equilibrium; and (b) fishing-effort responses in which fishing remains profitable over the entire season, hence, allowing fishing fleets to maintain a high fishing-effort throughout the season. Analyzing long-term within-year catch and effort data, we found that both Gulf of Mexico shrimp and North Territory giant crab fisheries are likely currently at healthy interannual bionomic equilibria, while certain socioeconomic drivers enable the Kuwait shrimp fishery to maintain high effort through the entire shrimping season. Our findings suggest that input controls are less effective in short-lived invertebrate fisheries that exhibit fishing-effort proportional to declining stock abundance. Conversely, if not regulated, the abundance-insensitive fishing-effort response could pose biological risks and habitat destruction. Therefore, we emphasize that in common-property seasonal fisheries, fishing-effort responses be scrutinized to distinguish factors that might undermine resource sustainability.
1. Introduction Invertebrate fisheries have been rapidly growing over the past four decades due to their increasing socioeconomic significance (Berkes et al., 2006; Sethi et al., 2010). Short-lived invertebrates such as shrimps, crabs, and squids, exhibit striking seasonal rhythm of abundance due to their high growth and natural mortality; as a result, highly seasonal fisheries arise from harvesting a single or double recruiting cohorts each year (e.g., Basson et al., 1996; Hay and Calogeras, 2000; Dichmont et al., 2003; McAllister et al., 2004; Diamond, 2005; Chen et al., 2007). However, such life-history characteristics, in addition to challenges in the aging process, have rendered most invertebrate fisheries poorly assessed and unregulated (Anderson et al., 2011; Punt et al., 2013). Understanding the interaction between fishing-effort and species in the linked fisher/fish system has been emphasized by many studies (Wilen, 1979; Hilborn, 1985; Mackinson et al., 1997; Branch et al., 2006). Such recognition is key for an effective fisheries management: at
∗
least some fisheries regulations directly influence the dynamics of fishing-effort rather than fish stocks. Consequently, ignoring these response dynamics will lead to bad predictions as surely as misunderstandings about the biology of the species. Central to the topic of seasonal effort responses is the concept of interannual bionomic equilibrium: absent management intervention, fleet dynamics may result in balanced year-to-year harvesting due to decreasing income per time fishing as biomass declines, without endangering the sustainability of the stock. Under certain circumstances, however, this equilibrium can be at a pathologically low stock size, notably when harvest removal become either independent of the stock size, or too high at all stock sizes. Such pattern is typically observed under three circumstances. First, market demand can drive the fishery to exert high efforts even when the stock is small; in this case, the payoff rate is expected to be higher than the cost of the entire fishing operation even at very low catch rates (see Wooster (1992) for the infamous case of the red king crab). Second, in a multi-species fishery where the incidental capture of a given stock suffers from a fishing-effort level that is independent of
Corresponding author. E-mail address: [email protected] (A. Ben-Hasan).
https://doi.org/10.1016/j.ocecoaman.2018.08.019 Received 13 March 2018; Received in revised form 11 June 2018; Accepted 17 August 2018 0964-5691/ © 2018 Elsevier Ltd. All rights reserved.
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the commercial fishery. The giant mud crab fishery is managed by the implementation of a suite of input measures such as restrictions from catching newly-molted mud crabs, minimum sizes, and fishing licenses. The fishing season is divided into wet season (April–October) and dry season (November–March), with the highest catches occurring between March and June (Meynecke et al., 2012).
the stock size (e.g., the incidental catch of billfish in the longline tuna and swordfish fisheries as reported by Graves et al., 2002). This case is exacerbated when the incidental capture occurs at a higher rate than the stock can withstand, causing severe stock size declines. The third circumstance involves the life-history characteristics of the harvested stock, particularly, the aggregation behavior either for spawning (e.g., cod) or other purposes such as defense (e.g., herring). As these fish stocks are depleted, they show a behavior known as range collapse; that is, their geographical distribution contracts (Walters and Maguire, 1996; Mackinson et al., 1997; Burgess et al., 2017). In this situation, fishers can still catch the same amount of fish even if the stock size is severely reduced, without a significant change in the amount of fishingeffort (or fishing cost). There is limited information in the literature about the topic of fishing-effort response dynamics for short-lived invertebrate fisheries. Yet, it has been observed that the effort dynamics in, at least, some of these kinds of fisheries are consistent with, i.e., should result in, an interannual bionomic equilibrium. For instance, Walters and Martell (2004) used Australia's shrimp and rock lobster fisheries to demonstrate that those fisheries are essentially self-regulated (i.e., the harvesting activity responds smoothly to changes in stock sizes and harvest costs), so that effort ends up being far lower than the limits imposed by input regulatory measures (license limitation, seasonal closures). However, Walters and Martell (2004) emphasized that: (a) latent effort, or the difference between potential and realized effort, could undermine the resource sustainability whenever it is economically viable; and (b) input measures assist in stabilizing the interannual bionomic equilibrium. Further examples are given here by examining the Gulf of Mexico (GOM) shrimp fishery and the North Territory Gulf of Carpentaria (NT) giant mud crab fishery. Also, we investigate the Kuwait shrimp fishery to demonstrate a case where fishing-effort is persistently high over the season due to certain socioeconomic drivers that generate a lack of feedback between fishing-effort and stock abundance. First, we provide a brief description of each fishery. Then, we present the main findings and delve into the discussion of the results and implications for management.
2.3. Kuwait shrimp fishery The exploitation of Kuwait shrimp started fifty years ago when several industrial trawlers were transferred from the U.S. Gulf of Mexico to Kuwait (Kristjonsson, 1967). Currently, the shrimp fishery is considered the most valuable fishery in Kuwait, accounting for 35% of the total landed yearly value (Al-Husaini et al., 2015). The fishery is supported by government subsidies and characterized by cheap labor wages and low fuel prices. Furthermore, an important demographic aspect of Kuwait shrimp fishery is that all shrimp fishers are foreigners, and by law, they can't own ships and must have a Kuwaiti sponsor. A direct consequence of such law is that very often ship owners demand a fixed payment from the fishers at the end of the shrimping season. Historically, Kuwait shrimp fishery experienced rapid growth in fishing-effort attracted by high yields, reaching a peak of about 5000 tonnes in the late 1980s. Since the Gulf War to the present, the shrimp fishery is undergoing a constant fishing-effort increase while maintaining an average shrimp production of around 2000 tonnes (AlHusaini et al., 2015). The shrimping season starts in August where fishing fleets are only allowed to fish in international waters. In September, the fishing fleets are permitted to start fishing in Kuwait's territorial waters. The most abundant and sought-after penaeid species in Kuwait is the tiger shrimp (Penaeus semisulcatus) with an average contribution of 60% to the overall annual catch. Therefore, the management of the shrimp fishery is based on the biology of P. semisulcatus (Bishop et al., 2001). Before 1979, Kuwait's shrimp fishery lacked basic regulations such as seasonal closures. However, given the increasing commercial importance of the shrimp fishery, several input controls have since been imposed to buffer against overexploitation including fishing licenses (without entry limits), seasonal closures (5–7 months) and closed areas (3 miles away from shore). Yet, factors such as fleet overcapacity and absence of enforcement have contributed to unsustainable declines in stock biomass (Chen et al., 2007; Al-Husaini et al., 2015).
2. Description of the fisheries 2.1. Gulf of Mexico shrimp fishery Trawling for shrimp started in 1917 off Louisiana State, where the catches increased from 15,000 tonnes in 1918 to a maximum of 130,000 tonnes in 1986 (Diamond, 2005). In 2015, shrimp landings amounted to 89,000 tonnes with a total value of $340 million, forming one of the most valuable fisheries in the United States (NMFS, 2015). The fishery mainly exploits three shrimp species: the brown shrimp (Farfantepenaeus aztecus), which represents the main species; the white shrimp (Litopenaeus setiferus); and the pink shrimp (Farfantepenaeus duorarum). The GOM shrimp fishery is regulated with a suite of input controls including limited licenses and several license buybacks in the Texas fishery, gear restrictions, seasonal closures and inshore-offshore designations and mandated bycatch reductions devices and turtle excluders.
3. Material and methods We used monthly catch and effort data to examine how within-year efforts respond to changes in stock abundance during fishing seasons (Table 1). To reconstruct the biomass trends for GOM shrimp, NT giant mud crab and Kuwait shrimp stocks, we applied a seasonal age-structured (monthly ages) model while assuming an age-vulnerability schedule and fishing mortality rate proportional to fishing-effort (using Table 1 Data used in the analysis of fishing-effort response dynamics for Gulf of Mexico (GOM) shrimp fishery, Northern Territory (NT) giant mud crab fishery and Kuwait shrimp fishery.
2.2. Northern Territory giant mud crab fishery The giant mud crab (Scylla serrata) is the most common crab species caught in the NT waters (Hay and Calogeras, 2000). This fishery started in the early 1980s and both catch and effort built to a peak in the late 1990s, but recently showed declining trends that have raised suspicions about the status of the stock. Several fisheries are involved in exploiting the giant mud crab stock including recreational, indigenous and commercial sectors, but the commercial fishery harvests about 90–95% of the total catch using traps as the sole fishing gear (Grubert et al., 2013). Consequently, catch and effort statistics are primarily collected from 34
Fishery
Data type (unit)
Data timeseries
Data timestep
Data source
GOM shrimp fishery
Catch rate (ton/days)
1960–2006
Monthly
NT giant mud crab fishery Kuwait shrimp fishery
Catch rate (kg/potlifts) Catch rate (ton/boatdays)
1983–2010
Monthly
1991–2012
Monthly
Louton, (thesis in prep.) Grubert et al. (2013) Al-Foudari et al. (2015)
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where S is the survival rate from natural mortality M (S = e−M ); and Ft , m is the fishing mortality rate in month m and year t calculated as the product of the effort (Et , m ) in month m and year t and the constant of proportionality (q ). We predicted catches (C ) using the seasonal catch equation:
Similar fishing-effort responses were observable in the NT giant mud crab fishery: the fishing-effort sharply plummeted over most seasons as the NT giant mud crab biomass declined (Fig. 2). However, the longterm fishing-effort level showed a steady build-up over the years (Fig. 2 (A)—(C)), unlike that in the GOM shrimp fishery which slightly increased with time (Fig. 1). Additionally, the NT giant mud crab biomass fluctuated between an average of 28 tonnes in 1983–1989 (Fig. 2 (A)) and 46 tonnes during 2000–2010 (Fig. 2 (C)). Kuwait shrimp fishery, however, showed different effort dynamics than that examined in both GOM shrimp and giant crab fisheries: the fishing fleets exerted high fishing-effort over the months for most seasons (Fig. 3). For example, the last month of most shrimping seasons indicated either higher or slightly lower fishing-effort level than the previous months in the season; consequently, driving shrimp biomass to very low levels, often to less than 1000 t (Fig. 3 (A) and (B)). Furthermore, Fig. 4 illustrates Kuwait's historical annual effort dynamics, where the fishers worked harder (i.e., expended more effort) in low biomass years (e.g., 2009–2012) so as to try and attain a certain catch target.
Ct , m = (1 − e−Ft, m) Bt , m
5. Discussion and implications for management
the same basic structure of the seasonal age-structured model described in Ben-Hasan et al., 2018). We estimated the vulnerable biomass (Bt , m ) in month m and year t using the concept of “incidence functions” (described in detail in Walters and Martell (2004)):
Bt , m =
∑ Na,(t,m) wa va
(1)
where Na,(t , m) denotes numbers-at-age a (in months) and months m and year t; wa is the mean weight-at-age; and va is fixed relative vulnerability-at-age which assumed to follow a logistic function for all case studies. We modelled numbers-at-age a (in months) and months m and year t using the survival equation:
Na + 1,(t , m + 1) = Na,(t , m) S (1 − va Ft , m)
(2)
(3)
The seasonal age-structured model assumes Beverton-Holt relationship of the form:
R1,(t , m + 1) =
αGt , m rm (xt − 1) e 1 + βGt , m
Like many seasonal fisheries, the case studies investigated here implement many input regulations to achieve certain biological and socioeconomic objectives: (a) season opening dates set to avoid growth overfishing (i.e., catching the shrimp before they reach optimum size); (b) in cases like the NT giant mud crabs, size limits to avoid growth overfishing and/or sex-selective discarding of females to protect egg production; (c) license limitations to assure better income per fisher; (d) mesh size and escape gap regulations to reduce bycatch; and closures related to particular species (e.g., banana vs. tiger shrimps in the Gulf of Carpentaria). The analysis of the fishing-effort responses in the GOM shrimp and NT giant mud crab fisheries showed that high fishing-effort is observed initially, then declines enough by the end of the designated fishing season. Ward and Sutinen (1994) studied the entry-exit behavior of the GOM shrimp fishing fleet and reported that fishing costs and exvessel prices are among the highest factors impacting the entry-exit decisions. Likewise, the seasonal response pattern of NT giant mud crab fishery exhibited harvest removals sensitive to the level of stock abundance. Our results suggest that for fisheries that exhibit fishingeffort response dynamics similar to GOM shrimp and NT giant mud crab fisheries, implementing input measures could be useful in stabilizing the interannual bionomic equilibrium (e.g., control peak effort), rather than effectively manage fisheries to achieve the overall biological and socioeconomic objectives. Nonetheless, input regulations are in fact necessary when the cost of fishing is low enough that absent regulations, fishers would drive the spawning stock too low to produce strong recruitment for the next season. For example, in open-access resources, Burgess et al. (2017) drew attention that stable fishing costs along with increasing high prices of the exploited species could amplify extinction risks. That biological risk could well be present in Kuwait shrimp fishery where costs of fishing are low, and market prices of shrimp are escalating (Kuwait's Central Statistical Office, 2008–2014). The persistent high fishing-effort in Kuwait shrimp fishery could be driven by several socioeconomic factors: direct governmental subsidies, low fishers' wages whom mostly come from Egypt, Bangladesh and India, and low fuel prices as Kuwait is an oil-rich country; all facilitate low fishing costs (Harry, 2007). Consequently, fishing remains profitable for all vessels over the whole season, allowing them to continue to operate even at low stock abundance. Additionally, the inference obtained from Fig. 4 might support anecdotal information that boat owners demand fixed payment from the expatriate fishers, which is collected at the end of the shrimping season, forcing them to keep fishing. Several studies reported that Kuwait shrimp stock is overfished and experiencing high overfishing (Chen et al., 2007; Al-Husaini et al., 2015). Moreover, since shrimp is harvested in Kuwait exclusively by
(4)
where α is the maximum number of recruits produced; Gt , m is the biomass in month m and year t (Gt , m = ∑ Na,(t , m) wa ); rm is the proportion of individuals reproducing in month m , where rm = 1 for the month (m = 1…12 ) of peak seasonal recruitment; β is density dependence; and e (xt − 1) represents the interannual variation in recruitment ( x t = 1 is the average relative recruitment). Both the timing of peak recruitment and the interannual variation in recruitment are estimated by allowing the model fitting procedure to vary rm and x t , respectively. We determined the stock-recruitment parameters (α and β ) using the initial parameters, unfished biomass (Bo ) and compensation ratio (CR ), and multiple incidence functions including unfished egg-per-recruit ∅ Eo = ∑ lx a fa (where lx a is the unfished survivorship to age a , lx a = 1, lx a = lx a − 1 S for a > 1), unfished biomass-per-recruit (∅ Bo = ∑ lx a wa va ) and unfished B recruitment (Ro = ∅ o ): Bo
CR α= ∅ Eo β=
CR−1 Ro ∅ Eo
(5)
(6)
We estimated the initial parameters (Bo and CR ), constant of proportionality q , proportion of reproducing individuals rm , and relative recruitment x t by fitting the assessment model using a sum of squares (negative log-likelihood) criterion assuming log-normally distributed observation errors (Walters and Martell, 2004). To demonstrate how Kuwait shrimp fleets are attempting to achieve some overall seasonal catch, we calculated the total effort necessary to attain a target catch (C ) (Walters and Martell, 2004):
Et =
C q × Bt
(7)
where Et is the fishing-effort (boat-days) in year t; and Bt is the biomass in year t estimated from Eq. (1). 4. Results Within-year effort dynamics of the GOM shrimp fishery showed that fishing-effort was parallel to the biomass of the shrimp stock (Fig. 1). This is demonstrated by the consistent pattern of the fishing-effort which increases around May–June, reach a peak in August–October, then plummets as the shrimp biomass decreases (Fig. 1 (A)—(E)). 35
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Fig. 1. Within-year observed effort and estimated biomass for the Gulf of Mexico shrimp fishery. (A) monthly observed effort and estimated biomass for the period 1960–1969; (B) monthly observed effort and estimated biomass for the period 1970–1979; (C) monthly observed effort and estimated biomass for the period 1980–1989; (D) monthly observed effort and estimated biomass for the period 1990–1999; (E) monthly observed effort and estimated biomass for the period 2000–2006.
Fig. 2. Within-year observed effort and estimated biomass for the Northern Territory giant mud crab fishery. (A) monthly observed effort and estimated biomass for the period 1983–1989; (B) monthly observed effort and estimated biomass for the period 1990–1999; (C) monthly observed effort and estimated biomass for the period 2000–2010.
money from fuel tax, for example, can be reinvested in the shrimp fishery for conducting research and management purposes, thus maintaining the money in the fishery. We hope that our study could initiate more detailed investigations into Kuwait shrimp fishery case which consider: (a) the economic factors such as harvesting costs and the amount of subsidies offered; and (b) societal factors including the demographic characteristics and the relationship between boat owners and expatriate fishers. We were able to identify two distinct fishing-effort responses in seasonal fisheries: fishing-effort response that is apparently parallel to
bottom trawling, expending such constant high effort throughout the season facilitates habitat destruction and incidental capture of charismatic species (Tillin et al., 2006; Anderson et al., 2011). Indeed, a combination of socioeconomic factors such as “bad” subsidies (as classified by Khan et al., 2006; Sumaila et al., 2010), low fuel prices and an expatriate labor force being demanded a fixed payment are undermining the sustainability of Kuwait shrimp stock. We suggest fishery subsidies reform, imposing fuel tax and regulating the owner-fisher relationship. These recommendations need not be understood as burdening the livelihoods that depend on the fishery; instead, taxpayer 36
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Fig. 4. Annual effort dynamics in Kuwait shrimp fishery. Dots and numbers indicate the observed total effort and years, respectively. The solid line represents the predicted total effort to achieve the target catch in each season.
species abundance leading to “healthy” bionomic equilibrium, and fishing-effort response that is independent of stock size, likely to result in overexploitation. However, these two types of effort dynamics need not be ubiquitous; rather, they might represent the far ends of the spectrum, where fishing-effort responses of other seasonal fisheries may fall anywhere in between. Further, we have investigated the effortbiomass dynamics by examining changes in the estimated biomass as a function of fishing-effort over time. Such approach is exploratory; an alternative approach would be to conduct time series analyses using statistical and empirical techniques to explain, for example, the periodicity and potential lags anticipated in the effort-biomass relationship (Camp et al., 2016). Acknowledgment We would like to thank three anonymous reviewers for their constructive comments. References Al-Foudari, H., Bishop, J.M., Chen, W., Alsaffar, A.H., Al-Said, T., Al-Baz, A., Ben-Hasan, A., 2015. A Comprehensive Management Strategy for the Long-term Sustainability of Kuwait's Shrimp Stocks, Final Report (FM047C). Kuwait Institute for Scientific Research. Al-Husaini, M., Bishop, J.M., Al-Foudari, H.M., 2015. “A Review of the Status and Development of Kuwait's Fisheries.” Marine Pollution Bulletin. Elsevier. http:// www.sciencedirect.com/science/article/pii/S0025326X15004737. Anderson, S.C., Flemming, J.M., Watson, R., Lotze, H.K., 2011. Rapid global expansion of invertebrate fisheries: trends, drivers, and ecosystem effects. PLoS One 6 (3), e14735. Basson, M., Beddington, J.R., Crombie, J.A., Holden, S.J., Purchase, L.V., Tingley, G.A., 1996. Assessment and management techniques for migratory squid stocks: the Illex argentines fishery in the Southwest Atlantic as an example. Fish. Res. 28, 3–27. Ben-Hasan, A., Walters, C., Christensen, V., Al-Husaini, M., Al-Foudari, H., 2018. Is reduced freshwater flow in tigris-euphrates rivers driving fish recruitment changes in the Northwestern Arabian Gulf? Mar. Pollut. Bull. 129 (1), 1–7. Berkes, F., Hughes, T.P., Steneck, R.S., Wilson, J.A., Bellwood, D.R., Crona, B., Folke, C.,
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Press, New Jersey. Ward, J.M., Sutinen, J.G., 1994. Vessel entry–exit behavior in the Gulf of Mexico shrimp fishery. Am. J. Agric. Econ. 76, 916–923. Wilen, J.E., 1979. Fisherman behavior and the design of efficient fisheries regulation programs. J. Fish. Res. Board Can. 36 (7) NRC Research Press: 855–58. Wooster, W.S., 1992. King crab dethroned. In: Glantz, M.H. (Ed.), Climate, Variability, Climate Change and Fisheries. Cambridge Univ. Press, New York, NY, pp. 14–30.
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Fishing-effort response dynamics in fisheries for short-lived invertebrates a,∗
a
a
a
a
A. Ben-Hasan , C. Walters , R. Louton , V. Christensen , U.R. Sumaila , H. Al-Foudari
T
b
a
Institute for the Oceans and Fisheries, University of British Columbia, Vancouver, BC, V6T 1Z4, Canada Ecosystem-based Management of Marine Resources, Environment and Life Sciences Research Center, Kuwait Institute for Scientific Research, PO Box 24885, 13109, Safat, Kuwait b
A R T I C LE I N FO
A B S T R A C T
Keywords: Fishing-effort responses Interannual bionomic equilibrium Invertebrate fisheries
In complex dynamic systems like fisheries, recognizing fishing-effort responses is as critical as understanding the biology of the exploited species for making sensible management decisions. In highly seasonal fisheries, it is theoretically possible for an “interannual bionomic equilibrium” to develop under open-access, where fleet dynamics may result in balanced year-to-year harvesting due to decreasing income per time fishing as biomass declines, without endangering the sustainability of the stock. However, in some conditions, this interannual bionomic equilibrium can be pathologically low leading to overfishing and amplification of extinction risks. Here we draw three cases from short-lived and fast-growing invertebrate fisheries to illustrate two distinct effort response dynamics: (a) fishing-effort responses that lead to healthy interannual bionomic equilibrium; and (b) fishing-effort responses in which fishing remains profitable over the entire season, hence, allowing fishing fleets to maintain a high fishing-effort throughout the season. Analyzing long-term within-year catch and effort data, we found that both Gulf of Mexico shrimp and North Territory giant crab fisheries are likely currently at healthy interannual bionomic equilibria, while certain socioeconomic drivers enable the Kuwait shrimp fishery to maintain high effort through the entire shrimping season. Our findings suggest that input controls are less effective in short-lived invertebrate fisheries that exhibit fishing-effort proportional to declining stock abundance. Conversely, if not regulated, the abundance-insensitive fishing-effort response could pose biological risks and habitat destruction. Therefore, we emphasize that in common-property seasonal fisheries, fishing-effort responses be scrutinized to distinguish factors that might undermine resource sustainability.
1. Introduction Invertebrate fisheries have been rapidly growing over the past four decades due to their increasing socioeconomic significance (Berkes et al., 2006; Sethi et al., 2010). Short-lived invertebrates such as shrimps, crabs, and squids, exhibit striking seasonal rhythm of abundance due to their high growth and natural mortality; as a result, highly seasonal fisheries arise from harvesting a single or double recruiting cohorts each year (e.g., Basson et al., 1996; Hay and Calogeras, 2000; Dichmont et al., 2003; McAllister et al., 2004; Diamond, 2005; Chen et al., 2007). However, such life-history characteristics, in addition to challenges in the aging process, have rendered most invertebrate fisheries poorly assessed and unregulated (Anderson et al., 2011; Punt et al., 2013). Understanding the interaction between fishing-effort and species in the linked fisher/fish system has been emphasized by many studies (Wilen, 1979; Hilborn, 1985; Mackinson et al., 1997; Branch et al., 2006). Such recognition is key for an effective fisheries management: at
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least some fisheries regulations directly influence the dynamics of fishing-effort rather than fish stocks. Consequently, ignoring these response dynamics will lead to bad predictions as surely as misunderstandings about the biology of the species. Central to the topic of seasonal effort responses is the concept of interannual bionomic equilibrium: absent management intervention, fleet dynamics may result in balanced year-to-year harvesting due to decreasing income per time fishing as biomass declines, without endangering the sustainability of the stock. Under certain circumstances, however, this equilibrium can be at a pathologically low stock size, notably when harvest removal become either independent of the stock size, or too high at all stock sizes. Such pattern is typically observed under three circumstances. First, market demand can drive the fishery to exert high efforts even when the stock is small; in this case, the payoff rate is expected to be higher than the cost of the entire fishing operation even at very low catch rates (see Wooster (1992) for the infamous case of the red king crab). Second, in a multi-species fishery where the incidental capture of a given stock suffers from a fishing-effort level that is independent of
Corresponding author. E-mail address: [email protected] (A. Ben-Hasan).
https://doi.org/10.1016/j.ocecoaman.2018.08.019 Received 13 March 2018; Received in revised form 11 June 2018; Accepted 17 August 2018 0964-5691/ © 2018 Elsevier Ltd. All rights reserved.
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the commercial fishery. The giant mud crab fishery is managed by the implementation of a suite of input measures such as restrictions from catching newly-molted mud crabs, minimum sizes, and fishing licenses. The fishing season is divided into wet season (April–October) and dry season (November–March), with the highest catches occurring between March and June (Meynecke et al., 2012).
the stock size (e.g., the incidental catch of billfish in the longline tuna and swordfish fisheries as reported by Graves et al., 2002). This case is exacerbated when the incidental capture occurs at a higher rate than the stock can withstand, causing severe stock size declines. The third circumstance involves the life-history characteristics of the harvested stock, particularly, the aggregation behavior either for spawning (e.g., cod) or other purposes such as defense (e.g., herring). As these fish stocks are depleted, they show a behavior known as range collapse; that is, their geographical distribution contracts (Walters and Maguire, 1996; Mackinson et al., 1997; Burgess et al., 2017). In this situation, fishers can still catch the same amount of fish even if the stock size is severely reduced, without a significant change in the amount of fishingeffort (or fishing cost). There is limited information in the literature about the topic of fishing-effort response dynamics for short-lived invertebrate fisheries. Yet, it has been observed that the effort dynamics in, at least, some of these kinds of fisheries are consistent with, i.e., should result in, an interannual bionomic equilibrium. For instance, Walters and Martell (2004) used Australia's shrimp and rock lobster fisheries to demonstrate that those fisheries are essentially self-regulated (i.e., the harvesting activity responds smoothly to changes in stock sizes and harvest costs), so that effort ends up being far lower than the limits imposed by input regulatory measures (license limitation, seasonal closures). However, Walters and Martell (2004) emphasized that: (a) latent effort, or the difference between potential and realized effort, could undermine the resource sustainability whenever it is economically viable; and (b) input measures assist in stabilizing the interannual bionomic equilibrium. Further examples are given here by examining the Gulf of Mexico (GOM) shrimp fishery and the North Territory Gulf of Carpentaria (NT) giant mud crab fishery. Also, we investigate the Kuwait shrimp fishery to demonstrate a case where fishing-effort is persistently high over the season due to certain socioeconomic drivers that generate a lack of feedback between fishing-effort and stock abundance. First, we provide a brief description of each fishery. Then, we present the main findings and delve into the discussion of the results and implications for management.
2.3. Kuwait shrimp fishery The exploitation of Kuwait shrimp started fifty years ago when several industrial trawlers were transferred from the U.S. Gulf of Mexico to Kuwait (Kristjonsson, 1967). Currently, the shrimp fishery is considered the most valuable fishery in Kuwait, accounting for 35% of the total landed yearly value (Al-Husaini et al., 2015). The fishery is supported by government subsidies and characterized by cheap labor wages and low fuel prices. Furthermore, an important demographic aspect of Kuwait shrimp fishery is that all shrimp fishers are foreigners, and by law, they can't own ships and must have a Kuwaiti sponsor. A direct consequence of such law is that very often ship owners demand a fixed payment from the fishers at the end of the shrimping season. Historically, Kuwait shrimp fishery experienced rapid growth in fishing-effort attracted by high yields, reaching a peak of about 5000 tonnes in the late 1980s. Since the Gulf War to the present, the shrimp fishery is undergoing a constant fishing-effort increase while maintaining an average shrimp production of around 2000 tonnes (AlHusaini et al., 2015). The shrimping season starts in August where fishing fleets are only allowed to fish in international waters. In September, the fishing fleets are permitted to start fishing in Kuwait's territorial waters. The most abundant and sought-after penaeid species in Kuwait is the tiger shrimp (Penaeus semisulcatus) with an average contribution of 60% to the overall annual catch. Therefore, the management of the shrimp fishery is based on the biology of P. semisulcatus (Bishop et al., 2001). Before 1979, Kuwait's shrimp fishery lacked basic regulations such as seasonal closures. However, given the increasing commercial importance of the shrimp fishery, several input controls have since been imposed to buffer against overexploitation including fishing licenses (without entry limits), seasonal closures (5–7 months) and closed areas (3 miles away from shore). Yet, factors such as fleet overcapacity and absence of enforcement have contributed to unsustainable declines in stock biomass (Chen et al., 2007; Al-Husaini et al., 2015).
2. Description of the fisheries 2.1. Gulf of Mexico shrimp fishery Trawling for shrimp started in 1917 off Louisiana State, where the catches increased from 15,000 tonnes in 1918 to a maximum of 130,000 tonnes in 1986 (Diamond, 2005). In 2015, shrimp landings amounted to 89,000 tonnes with a total value of $340 million, forming one of the most valuable fisheries in the United States (NMFS, 2015). The fishery mainly exploits three shrimp species: the brown shrimp (Farfantepenaeus aztecus), which represents the main species; the white shrimp (Litopenaeus setiferus); and the pink shrimp (Farfantepenaeus duorarum). The GOM shrimp fishery is regulated with a suite of input controls including limited licenses and several license buybacks in the Texas fishery, gear restrictions, seasonal closures and inshore-offshore designations and mandated bycatch reductions devices and turtle excluders.
3. Material and methods We used monthly catch and effort data to examine how within-year efforts respond to changes in stock abundance during fishing seasons (Table 1). To reconstruct the biomass trends for GOM shrimp, NT giant mud crab and Kuwait shrimp stocks, we applied a seasonal age-structured (monthly ages) model while assuming an age-vulnerability schedule and fishing mortality rate proportional to fishing-effort (using Table 1 Data used in the analysis of fishing-effort response dynamics for Gulf of Mexico (GOM) shrimp fishery, Northern Territory (NT) giant mud crab fishery and Kuwait shrimp fishery.
2.2. Northern Territory giant mud crab fishery The giant mud crab (Scylla serrata) is the most common crab species caught in the NT waters (Hay and Calogeras, 2000). This fishery started in the early 1980s and both catch and effort built to a peak in the late 1990s, but recently showed declining trends that have raised suspicions about the status of the stock. Several fisheries are involved in exploiting the giant mud crab stock including recreational, indigenous and commercial sectors, but the commercial fishery harvests about 90–95% of the total catch using traps as the sole fishing gear (Grubert et al., 2013). Consequently, catch and effort statistics are primarily collected from 34
Fishery
Data type (unit)
Data timeseries
Data timestep
Data source
GOM shrimp fishery
Catch rate (ton/days)
1960–2006
Monthly
NT giant mud crab fishery Kuwait shrimp fishery
Catch rate (kg/potlifts) Catch rate (ton/boatdays)
1983–2010
Monthly
1991–2012
Monthly
Louton, (thesis in prep.) Grubert et al. (2013) Al-Foudari et al. (2015)
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where S is the survival rate from natural mortality M (S = e−M ); and Ft , m is the fishing mortality rate in month m and year t calculated as the product of the effort (Et , m ) in month m and year t and the constant of proportionality (q ). We predicted catches (C ) using the seasonal catch equation:
Similar fishing-effort responses were observable in the NT giant mud crab fishery: the fishing-effort sharply plummeted over most seasons as the NT giant mud crab biomass declined (Fig. 2). However, the longterm fishing-effort level showed a steady build-up over the years (Fig. 2 (A)—(C)), unlike that in the GOM shrimp fishery which slightly increased with time (Fig. 1). Additionally, the NT giant mud crab biomass fluctuated between an average of 28 tonnes in 1983–1989 (Fig. 2 (A)) and 46 tonnes during 2000–2010 (Fig. 2 (C)). Kuwait shrimp fishery, however, showed different effort dynamics than that examined in both GOM shrimp and giant crab fisheries: the fishing fleets exerted high fishing-effort over the months for most seasons (Fig. 3). For example, the last month of most shrimping seasons indicated either higher or slightly lower fishing-effort level than the previous months in the season; consequently, driving shrimp biomass to very low levels, often to less than 1000 t (Fig. 3 (A) and (B)). Furthermore, Fig. 4 illustrates Kuwait's historical annual effort dynamics, where the fishers worked harder (i.e., expended more effort) in low biomass years (e.g., 2009–2012) so as to try and attain a certain catch target.
Ct , m = (1 − e−Ft, m) Bt , m
5. Discussion and implications for management
the same basic structure of the seasonal age-structured model described in Ben-Hasan et al., 2018). We estimated the vulnerable biomass (Bt , m ) in month m and year t using the concept of “incidence functions” (described in detail in Walters and Martell (2004)):
Bt , m =
∑ Na,(t,m) wa va
(1)
where Na,(t , m) denotes numbers-at-age a (in months) and months m and year t; wa is the mean weight-at-age; and va is fixed relative vulnerability-at-age which assumed to follow a logistic function for all case studies. We modelled numbers-at-age a (in months) and months m and year t using the survival equation:
Na + 1,(t , m + 1) = Na,(t , m) S (1 − va Ft , m)
(2)
(3)
The seasonal age-structured model assumes Beverton-Holt relationship of the form:
R1,(t , m + 1) =
αGt , m rm (xt − 1) e 1 + βGt , m
Like many seasonal fisheries, the case studies investigated here implement many input regulations to achieve certain biological and socioeconomic objectives: (a) season opening dates set to avoid growth overfishing (i.e., catching the shrimp before they reach optimum size); (b) in cases like the NT giant mud crabs, size limits to avoid growth overfishing and/or sex-selective discarding of females to protect egg production; (c) license limitations to assure better income per fisher; (d) mesh size and escape gap regulations to reduce bycatch; and closures related to particular species (e.g., banana vs. tiger shrimps in the Gulf of Carpentaria). The analysis of the fishing-effort responses in the GOM shrimp and NT giant mud crab fisheries showed that high fishing-effort is observed initially, then declines enough by the end of the designated fishing season. Ward and Sutinen (1994) studied the entry-exit behavior of the GOM shrimp fishing fleet and reported that fishing costs and exvessel prices are among the highest factors impacting the entry-exit decisions. Likewise, the seasonal response pattern of NT giant mud crab fishery exhibited harvest removals sensitive to the level of stock abundance. Our results suggest that for fisheries that exhibit fishingeffort response dynamics similar to GOM shrimp and NT giant mud crab fisheries, implementing input measures could be useful in stabilizing the interannual bionomic equilibrium (e.g., control peak effort), rather than effectively manage fisheries to achieve the overall biological and socioeconomic objectives. Nonetheless, input regulations are in fact necessary when the cost of fishing is low enough that absent regulations, fishers would drive the spawning stock too low to produce strong recruitment for the next season. For example, in open-access resources, Burgess et al. (2017) drew attention that stable fishing costs along with increasing high prices of the exploited species could amplify extinction risks. That biological risk could well be present in Kuwait shrimp fishery where costs of fishing are low, and market prices of shrimp are escalating (Kuwait's Central Statistical Office, 2008–2014). The persistent high fishing-effort in Kuwait shrimp fishery could be driven by several socioeconomic factors: direct governmental subsidies, low fishers' wages whom mostly come from Egypt, Bangladesh and India, and low fuel prices as Kuwait is an oil-rich country; all facilitate low fishing costs (Harry, 2007). Consequently, fishing remains profitable for all vessels over the whole season, allowing them to continue to operate even at low stock abundance. Additionally, the inference obtained from Fig. 4 might support anecdotal information that boat owners demand fixed payment from the expatriate fishers, which is collected at the end of the shrimping season, forcing them to keep fishing. Several studies reported that Kuwait shrimp stock is overfished and experiencing high overfishing (Chen et al., 2007; Al-Husaini et al., 2015). Moreover, since shrimp is harvested in Kuwait exclusively by
(4)
where α is the maximum number of recruits produced; Gt , m is the biomass in month m and year t (Gt , m = ∑ Na,(t , m) wa ); rm is the proportion of individuals reproducing in month m , where rm = 1 for the month (m = 1…12 ) of peak seasonal recruitment; β is density dependence; and e (xt − 1) represents the interannual variation in recruitment ( x t = 1 is the average relative recruitment). Both the timing of peak recruitment and the interannual variation in recruitment are estimated by allowing the model fitting procedure to vary rm and x t , respectively. We determined the stock-recruitment parameters (α and β ) using the initial parameters, unfished biomass (Bo ) and compensation ratio (CR ), and multiple incidence functions including unfished egg-per-recruit ∅ Eo = ∑ lx a fa (where lx a is the unfished survivorship to age a , lx a = 1, lx a = lx a − 1 S for a > 1), unfished biomass-per-recruit (∅ Bo = ∑ lx a wa va ) and unfished B recruitment (Ro = ∅ o ): Bo
CR α= ∅ Eo β=
CR−1 Ro ∅ Eo
(5)
(6)
We estimated the initial parameters (Bo and CR ), constant of proportionality q , proportion of reproducing individuals rm , and relative recruitment x t by fitting the assessment model using a sum of squares (negative log-likelihood) criterion assuming log-normally distributed observation errors (Walters and Martell, 2004). To demonstrate how Kuwait shrimp fleets are attempting to achieve some overall seasonal catch, we calculated the total effort necessary to attain a target catch (C ) (Walters and Martell, 2004):
Et =
C q × Bt
(7)
where Et is the fishing-effort (boat-days) in year t; and Bt is the biomass in year t estimated from Eq. (1). 4. Results Within-year effort dynamics of the GOM shrimp fishery showed that fishing-effort was parallel to the biomass of the shrimp stock (Fig. 1). This is demonstrated by the consistent pattern of the fishing-effort which increases around May–June, reach a peak in August–October, then plummets as the shrimp biomass decreases (Fig. 1 (A)—(E)). 35
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Fig. 1. Within-year observed effort and estimated biomass for the Gulf of Mexico shrimp fishery. (A) monthly observed effort and estimated biomass for the period 1960–1969; (B) monthly observed effort and estimated biomass for the period 1970–1979; (C) monthly observed effort and estimated biomass for the period 1980–1989; (D) monthly observed effort and estimated biomass for the period 1990–1999; (E) monthly observed effort and estimated biomass for the period 2000–2006.
Fig. 2. Within-year observed effort and estimated biomass for the Northern Territory giant mud crab fishery. (A) monthly observed effort and estimated biomass for the period 1983–1989; (B) monthly observed effort and estimated biomass for the period 1990–1999; (C) monthly observed effort and estimated biomass for the period 2000–2010.
money from fuel tax, for example, can be reinvested in the shrimp fishery for conducting research and management purposes, thus maintaining the money in the fishery. We hope that our study could initiate more detailed investigations into Kuwait shrimp fishery case which consider: (a) the economic factors such as harvesting costs and the amount of subsidies offered; and (b) societal factors including the demographic characteristics and the relationship between boat owners and expatriate fishers. We were able to identify two distinct fishing-effort responses in seasonal fisheries: fishing-effort response that is apparently parallel to
bottom trawling, expending such constant high effort throughout the season facilitates habitat destruction and incidental capture of charismatic species (Tillin et al., 2006; Anderson et al., 2011). Indeed, a combination of socioeconomic factors such as “bad” subsidies (as classified by Khan et al., 2006; Sumaila et al., 2010), low fuel prices and an expatriate labor force being demanded a fixed payment are undermining the sustainability of Kuwait shrimp stock. We suggest fishery subsidies reform, imposing fuel tax and regulating the owner-fisher relationship. These recommendations need not be understood as burdening the livelihoods that depend on the fishery; instead, taxpayer 36
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Fig. 4. Annual effort dynamics in Kuwait shrimp fishery. Dots and numbers indicate the observed total effort and years, respectively. The solid line represents the predicted total effort to achieve the target catch in each season.
species abundance leading to “healthy” bionomic equilibrium, and fishing-effort response that is independent of stock size, likely to result in overexploitation. However, these two types of effort dynamics need not be ubiquitous; rather, they might represent the far ends of the spectrum, where fishing-effort responses of other seasonal fisheries may fall anywhere in between. Further, we have investigated the effortbiomass dynamics by examining changes in the estimated biomass as a function of fishing-effort over time. Such approach is exploratory; an alternative approach would be to conduct time series analyses using statistical and empirical techniques to explain, for example, the periodicity and potential lags anticipated in the effort-biomass relationship (Camp et al., 2016). Acknowledgment We would like to thank three anonymous reviewers for their constructive comments. References Al-Foudari, H., Bishop, J.M., Chen, W., Alsaffar, A.H., Al-Said, T., Al-Baz, A., Ben-Hasan, A., 2015. A Comprehensive Management Strategy for the Long-term Sustainability of Kuwait's Shrimp Stocks, Final Report (FM047C). Kuwait Institute for Scientific Research. Al-Husaini, M., Bishop, J.M., Al-Foudari, H.M., 2015. “A Review of the Status and Development of Kuwait's Fisheries.” Marine Pollution Bulletin. Elsevier. http:// www.sciencedirect.com/science/article/pii/S0025326X15004737. Anderson, S.C., Flemming, J.M., Watson, R., Lotze, H.K., 2011. Rapid global expansion of invertebrate fisheries: trends, drivers, and ecosystem effects. PLoS One 6 (3), e14735. Basson, M., Beddington, J.R., Crombie, J.A., Holden, S.J., Purchase, L.V., Tingley, G.A., 1996. Assessment and management techniques for migratory squid stocks: the Illex argentines fishery in the Southwest Atlantic as an example. Fish. Res. 28, 3–27. Ben-Hasan, A., Walters, C., Christensen, V., Al-Husaini, M., Al-Foudari, H., 2018. Is reduced freshwater flow in tigris-euphrates rivers driving fish recruitment changes in the Northwestern Arabian Gulf? Mar. Pollut. Bull. 129 (1), 1–7. Berkes, F., Hughes, T.P., Steneck, R.S., Wilson, J.A., Bellwood, D.R., Crona, B., Folke, C.,
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