Fisheries harvest management procedures under time-varying productivity

Fish stocks often show persistent changes in fish production with hypothesized causes including time-varying recruit parameters, natural mortality of mature fish, and somatic growth. Changes in fish productivity can affect biological reference points and the expected robustness of harvest strategies in meeting long-term conservation and economic objectives. Current assessment approaches commonly assume “stationarity” in stock-recruit parameters and implemented harvest strategies commonly lack consideration of changes in fish productivity. This dissertation covers three studies based on statistical analysis and simulation methods that aim to advance understanding of time-varying stock-recruit parameters and harvest control rules (HCRs) to cope with changes in fish production. Chapter 2 examined the performance of a Kalman filter algorithm that estimates dynamic state variables (e.g., changing stock-recruit parameters) from inaccurate observations obtained from stock assessment outputs. In Chapter 3, stochastic optimization in policy space (SOPS) was used to find an optimal HCR that is robust to changes in productivity under closed-loop simulation or management strategy evaluation (MSE). By using SOPS and a simple population dynamics model, this chapter examined optimal HCRs under different types of regime shifts (e.g., changes in carrying capacity and intrinsic growth rates). Chapter 4 examined empirical HCRs in an extreme situation of “non-stationarity” in stock-recruit parameters (i.e., a spasmodic recruitment pattern), using Atlantic redfish stocks as a case-study. The Kalman filter detected the overall trend in the stock-recruit parameters over time. Also, it was found that optimal HCRs depend on the timing of biomass estimates relative to harvest, observation error, intrinsic population growth, and regime shifts types. In the case of spasmodic stocks, it was found that simple HCRs that used data from well-designed fishery-independent resource surveys were capable of handling changes in productivity. The present dissertation shows the importance of well-designed fishery-independent surveys and simple HCRs for tackling changes in productivity and guidelines are provided for developing HCRs that are robust to the occurrence of such changes. The models and findings presented here can be applied to improve the management of other stocks that show persistent changes in fish productivity.Science, Faculty ofOceans and Fisheries, Institute for theGraduat

Estructura de tallas y proporción sexual de Anadara tuberculosa y Anadara similis en una reserva de manglares al noroeste de Ecuador

Shell length frequency distribution (SLFD) and sex ratio of a subpopulation of the mangrove cockles Anadara tuberculosa and Anadara similis were analyzed from natural beds in the Cayapas-Mataje Ecological Mangrove Reserve, Ecuador. The SLFD was different between sexes for both species; females were larger than males. Subpopulations of A. tuberculosa and A. similis were dominated by females throughout the year. The overall sex ratio was 2.6:1 (female: male), although with variations in the monthly samples, from 1.2:1 to 5.9:1 in A. tuberculosa and 1.9:1 to 5.3:1 in A. similis. Possibly, these results may be due to differential growth rate between sexes and environmental factors along the year

Species catch composition, length structure and reproductive indices of swordfish (Xiphias gladius) at Easter Island zone Composición de especies, estructura de tallas e índices reproductivos de pez espada (Xiphias gladius) en la zona de isla de Pascua

The species composition, size distribution, and gonadal index (GI) of swordfish (Xiphias gladius) are reported from catches obtained by longliners in the zone of Easter Island in the southeastern Pacific. Five cruises were made during the summer season between 2001 and 2006 completing a total of 150 fishing sets. A total of 3,781 fish belonging to 24 species were registered, of which 17 were commercial and 6 were bycatch. Of the total, 44.1%) were swordfish, 28.3%> sharks, 12.1% marlins, 8.4%> tunas and 7.1%o other teleosts. Amongst the sharks the most important species was the blue shark (Prionace glauca, 16.5%>) followed by the brown shark (Carcharhinus spp., 6.9%>). Amongst the tunas and istiophorids, striped marlin (Tetrapturus audax, 8.0%) and yellowfin tuna (Thunnus albacares, 4.9%) were also important (&#8805; 4% in number). The number of units by species, the nominal yield in number and weight and the gutted average weight are reported in addition to size statistics. The population structure of swordfish was dominated by males and differences in the average length between sexes were not found. Individuals with high values of GI were observed during every survey, mainly during December-February which suggests a reproductive area for this species. The blue shark showed a predominance of males, and differences in the average length between sexes were observed, possibly due to a spatial and temporal segregation.<br>Se reportan la composición de especies, estructura de talla, y el índice gonádico (IG) de pez espada (Xiphias gladius), desde capturas obtenidas por embarcaciones palangreras en la zona de isla de Pascua en el Pacífico suroriental. En el periodo estival entre 2001 y 2006, se realizaron cinco cruceros completando un total de 150 lances de pesca. Se registró un total de 3.781 peces pertenecientes a 24 especies, de las cuales 17 fueron comerciales y seis incidentales. Del total, 44,1% fue pez espada, 28,3%> tiburones, 12,1% marlines, 8,4%o atunes y 7,1% otros teleósteos. Entre los tiburones, la especie más relevante fue el tiburón azulejo (Prionace glauca, 16,5%o), seguido por el tiburón jaquetón (Carcharhinus spp., 6,9%). Entre los atunes e istiofóridos, el marlín rayado (Tetrapturus audax, 8,0%) y el atún aleta amarilla (Thunnus albacares, 4,9%) fueron también importantes (&#8805; 4% en número). Se reporta el número de ejemplares por especie, su rendimiento nominal en número y peso, y el peso promedio eviscerado, además de los estadísticos descriptivos de talla. La estructura poblacional de pez espada estuvo dominada por machos y no se registraron diferencias en las tallas medias entre sexos. Se observaron ejemplares con altos valores de IG durante todos los cruceros, principalmente durante diciembre-febrero, lo que sugiere un area reproductiva para esta especie. El tiburón azulejo mostró una predominancia de machos, y se observaron diferencias en las tallas medias entre sexos, posiblemente debido a una segregación espacial y temporal

Species catch composition, length structure and reproductive indices of swordfish (Xiphias gladius) at Easter Island zone

The species composition, size distribution, and gonadal index (GI) of swordfish (Xiphias gladius) are reported from catches obtained by longliners in the zone of Easter Island in the southeastern Pacific. Five cruises were made during the summer season between 2001 and 2006 completing a total of 150 fishing sets. A total of 3,781 fish belonging to 24 species were registered, of which 17 were commercial and 6 were bycatch. Of the total, 44.1%) were swordfish, 28.3%> sharks, 12.1% marlins, 8.4%> tunas and 7.1%o other teleosts. Amongst the sharks the most important species was the blue shark (Prionace glauca, 16.5%>) followed by the brown shark (Carcharhinus spp., 6.9%>). Amongst the tunas and istiophorids, striped marlin (Tetrapturus audax, 8.0%) and yellowfin tuna (Thunnus albacares, 4.9%) were also important (&#8805; 4% in number). The number of units by species, the nominal yield in number and weight and the gutted average weight are reported in addition to size statistics. The population structure of swordfish was dominated by males and differences in the average length between sexes were not found. Individuals with high values of GI were observed during every survey, mainly during December-February which suggests a reproductive area for this species. The blue shark showed a predominance of males, and differences in the average length between sexes were observed, possibly due to a spatial and temporal segregation

Intra-specific variability in life-history traits of Anadara tuberculosa (Mollusca: Bivalvia) in the mangrove ecosystem of the Southern coast of Ecuador

Anadara tuberculosa is one of the most important bivalves along the Western Pacific coast because of its commercial value. Nevertheless, the variability in growth, longlife span, natural mortality and reproductive parameters of this mangrove cockle has not yet been described. The aim of this study was to analyze these lifehistory traits in three areas of the Southern coast of Ecuador. Empirical and length-based methods were used to estimate these biological parameters. Body size data were collected from the commercial fishery between 2004 and 2011 in landing ports near to the Archipelago of Jambeli [Puerto Bolivar (PB), Puerto Jeli (PJ) and Puerto Hualtaco (PH)]. The von Bertalanffy growth parameters for combined sex were estimated between 70.87 to 93.45mm for L∞ and 0.22 to 0.80/year for k. The growth indices (Φ’) ranged from 3.17 to 3.85, while the overall growth performance (OGP) ranged from 5.03 to 5.82. The mean of long-life span (t max), size and age at maturity (L50% and t50%) were estimated in 7.71±2.53years, 39.13±2.24mm and 1.46±0.56years for PB; 9.51±2.85years, 37.78±1.95mm and 1.37±0.41years for PJ and 5.81±2.11years, 39.73±3.31mm and 0.94±0.41years for PH. Natural mortality (M) ranged from 0.46 to 1.28/year. We concluded that significant intra-specific variation was observed in a temporal scale in Φ’ and OGP indices as well as L50% and M. Therefore, temporal changes in these life-history traits should be taken into account when assessing the status of the mangrove cockle fishery

Management strategies for spasmodic stocks: a Canadian Atlantic redfish fishery case study

There exist few recommendations for managing stocks with spasmodic recruitment, despite such stocks being not uncommon. Management procedures (MPs), developed for two rockfishes (Sebastes mentella and Sebastes fasciatus) in eastern Canada, are recommended for setting catch limits during periods of high and low abundance. A well-designed fishery-independent trawl survey is essential to provide advance warning of strong recruitment events and project future recruitment. Under an “inventory management” strategy, a more appropriate aim in spasmodic stocks may be to maximize the number of years with “good catches,” instead of maximizing total catches, as is traditionally considered in management strategy evaluation (MSE). Following a spasmodic recruitment event, an empirical harvest control rule based on larger fish delays the harvest of large cohorts by a few years, targets more commercially valuable fish sizes, and reduces the risk of growth overfishing. Capped MPs produced longer periods of large catches than uncapped MPs. MPs allowed for low harvests during periods of low abundance, thus avoiding unnecessary hardship in the industry. MPs evaluated here could be good candidates for other stocks with similar or less extreme recruitment variability.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author