Population dynamics of benthic shellfisheries in the north of Chile: the role of denso-dependent processes

and oceanographic forcing

Marcelo M. Rivadeneira1, 2, Raúl Ulloa3, Adolfo Vargas4,5 & Cristian Hudson5

1Centro de Estudios Avanzados en Zonas Áridas (CEAZA) & Facultad de Ciencias del Mar, Universidad

Católica del Norte, P.O. Box 117, Coquimbo, Chile

2Center for Advanced Studies in Ecology and Biodiversity (CASEB), Departamento de Ecología Pontificia

Universidad Católica de Chile, Alameda 340, Santiago, Chile

3Centro Regional de Investigación Pesquera, Instituto Nacional de Pesca, Dirección General de

Investigación Pesquera Pacifico Norte. Miguel Alemán sur Nº605, C.P. 85400, Guaymas, Sonora, México

4Centro de Investigación y Desarrollo de Profesionales Marinos Pacífico Ltda. Vivar 1218, Iquique, Chile

5Programa Magíster en Ciencias Aplicadas mención Biología Pesquera, Departamento Ciencias del Mar,

Universidad Arturo Prat, P.O. Box 121, Iquique, Chile

Abstract:

The need of establish sustainable fishing policies clashes against the incapability of many models to

integrate mechanistically, yet in a simple way, the role played by biological (i.e. endogenous) and physical

(i.e. exogenous, oceanographic) forcing in the regulation of population dynamics. In this work we evaluate

the importance of endogenous and exogenous factors in the regulation of population dynamic in 10 benthic

species commercially exploited by the fishing artisanal fleet along northern coast of Chile. The information

was gathered from annual time series of CPUE (captures per unit of effort) between 1998 and 2005. The

population growth rate (i.e. interannual changes in the CPUE) was modeled as an autoregressive lineal

process, evaluating the importance of first-order lags (1 year) of CPUE, and oceanographic factors (southern

oscillation index, sea surface temperature, and upwelling index). In all species, the best models included the

effect of both endogenous and exogenous factors, explaining a high percentage of the variance of the

population growth rate (in average 88 %). These results support the idea that the studied populations,

despite being under an exploitation regime, show a strong regulation seemingly produced by the combined

effect of intraspecific interactions and physical forcing molding the abundance during the early stages of the

species. Our results highlight the importance of these kinds of models as a powerful tool to evaluate the

processes underlying the regulation of natural populations.

Key words: artisanal fisheries, Pacific South America, autoregressive model, El Niño, coastal upwelling

Resumen

La necesidad de establecer medidas de manejo pesquero sustentable se ve entorpecida por la incapacidad

de muchos modelos de incorporar mecanísticamente, pero aún de manera simple, el rol desempeñado por

forzantes biológicos (i.e. endógenos) y físicos (i.e. exógenos u oceanográficos) en la regulación de las

dinámicas poblacionales. En este trabajo evaluamos la existencia de regulación poblacional en 10 especies

bentónicas explotadas comercialmente por la flota pesquera artesanal en el norte grande de Chile. La

información fue obtenida a partir de series de tiempo anuales de capturas por unidad de esfuerzo (CPUE)

entre 1998 y 2005. Las tasas de crecimiento poblacional (i.e. cambios interanuales en la CPUE) fueron

modeladas como un proceso autoregresivo lineal, evaluándose la importancia de retrasos de primer orden

(1 año) en las capturas por unidad de esfuerzo y de factores oceanográficos (índice de oscilación del sur,

temperatura superficial del mar, e índice de surgencia costera). En todas las especies se verificó que los

mejores modelos incorporaron efectos denso-dependientes endógenos y exógenos, los cuales explicaron un

alto porcentaje de la varianza (en promedio 88%) de la tasa de crecimiento poblacional. Estos resultados

apoyan la idea de que las poblaciones estudiadas, a pesar de estar sometidas a un régimen de explotación,

exhiben una fuerte regulación, la que aparentemente estaría dada por interacciones intra-específicas y

forzamientos físicos que moldearían ciclos de vida tempranos de las especies. Los resultados aquí

obtenidos subrayan la utilidad de este tipo de modelos como valiosa herramienta para evaluar los procesos

subyacentes a regulación de las poblaciones naturales.

Palabras clave: pesquería artesanal, Pacífico de Sudamérica, modelo autoregresivo, biológicEl Niño,

surgencia costera

Introducción

The collapse of many fisheries along the global ocean has emphasized the urgent need to unveil the latest

processes and mechanisms that govern the dynamics of temporary space populations. In this sense, the

explicit incorporation of environment in the evaluation of fisheries is to be considered as relevant end to

achieve this objective (Wiff & Quiñones 2004). There are numerous examples that demonstrate the strong

dependence of the pelagic fisheries of the oceanographic conditions prevailing (Mantua et al. 1997, Yáñez et

al. 2001, Mantua & Hare 2002). However, the direct application and success of the conclusions reached

toward management measures is still in its infancy (Yañez et al. 2001). The need to implement better

models that make it possible to understand the dynamic fishing has led to the implementation of numerous

analytical perspective that explicitly or implicitly considered the role of the environment (e.g. , models based

on individuals, models of energy acquisition, analysis age/size structured, neural networks). However, its

application and implementation requires the generation of new and complex biological information, which is

not feasible in most cases. The scenario is even more complex in the case of benthic species of Chile, where

the bulk of the recent research has focused on descriptions bio-fishing (Defeo & Castile 1998, Castilla &

Defeo 2001, Leiva & Castile 2001).

The management recommendations have been based, in the last few years, in the implementation of marine

protected areas or areas of co-management fisheries (Fernandez & Castile 2005), however, those

recommendations underpinning success in a limited knowledge of the dynamic spatio-temporal system.

Even more, the very few models of management applied to benthic species in northern Chile that consider

the environment in an explicit manner (The Field 2002, Ortiz & Wolff 2002a, b, c), are of purely local area

and are methodologically difficult to implement (Why?). In a temporal context, fluctuations in the fisheries

have been associated with variations oceanographic, operants to macro-spatial scales and low-frequency

(Mantua et al. 1997, Chavez et al. 1999, Yáñez et al. 2001, Mantua & Hare 2002, Yáñez et al. 2002). In

most cases however, the fishing environmental analysis has been carried out mostly with a

phenomenological philosophy, for example leaving in evidence the importance of the sea surface

temperature (Freon & Yanez 1995) or of the turbulence (Yañez et al. 2001) In the interannual variations in

the catch of pelagic resources. However, this perspective does not account for the processes and/or

regulatory mechanisms that subyacerian to this correlation. Prospects more refined models based on

deterministic or stochastic majority have considered the role of endogenous factors (density-dependence),

but not of the environmental conditions in the population regulation (wiff & Quiñones 2004). The explicit

incorporation of endogenous and exogenous factors (environmental conditions) as determinants of

population dynamics has been developed widely in the environmental field in the last few decades (Lima et

al. 2001, Lima et al. 2002, Stenseth et al. 2002, Stenseth et al. 2003). Basically it states that the temporal

variations (e.g. , inter-annual) of the stocks would emerge as part of an internal dynamic, in which the built-in

bias toward the populations to expand (i.e. exponential growth) would be offset by density-dependent

processes (e.g. , intraspecific competition, competition inter-specific, predation).

In this framework, the role of environmental forcings (e.g. , the Child, transport offshore) would be made a

manifest through the dense structure-dependence on the population, and could be modeled explicitly, in

linear terms or non-linear (Stenseth et al. 2002). The action of both mechanisms and its time lag would be

causing the populations to vary in the time (short and/or medium-term), although in the long term, the

abundance of the population is stationary (do permanently stationary or determined by the load capacity of

the environment, which may also have different levels? ). Then, the existence of natural cycles in the

abundance of populations, evident also in benthic species, that could and should be considered explicitly in

the fisheries management plans. In the present study raises the explicit use of this approach of analysis is to

allow glimpse possible processes and mechanisms that govern temporary fluctuations in benthic fisheries in

the north of Chile.

In particular, it evaluates the adjustment autoregressive models that consider endogenous and exogenous

effects on the population dynamics of 10 species. This step would be key in the generation of effective

management measures for these resources.

Material y métodos

Obtaining time series of CPUE: information was obtained from a database of the follow-up project of

benthic fisheries executed by the Fisheries Development Institute of Chile (FIFG), where are documented

catches (kg) for various benthic resources in a total of 231 sites (backgrounds) distributed from Africa to the

south of Antofagasta (ca.18 ° -25 °S), covering an area of ca. 800 Km of coastline, between the years 1998-

2005. In order to obtain a synoptic view, chose to ignore trends space as well as temporal trends intra-

annual, by expressing the total annual catch as for each resource. As a measure of fishing effort is used the

number of trips made to that origin for that resource and year. Of the total of 30 species recorded only

analysis was performed with 10 of them, since they correspond to the 98% of this information (Table 1).

Obtaining of oceanographic information: synoptic oceanographic information was obtained in the form of

annual averages for the entire region considering the period 1998-2005. Three oceanographic variables

were analyzed: sea surface temperature (SST), index of Southern Oscillation (SOI), and index of upwelling

(UPW). It generated an average annual regional TSM using the information obtained for three main ports

(Arica, Iquique, Antofagasta) using in-situ measurements (CENDOHC-SHOA). The average annual value of

SOI and UPW (transport offshore Ekman) was obtained from time series available at NOAA, and were

recovered in your web site (http://www.pfeg.noaa.gov:16080/products/PFEL/) finally these values were

obtained by averaging the values of 3 stations that are available for each fishing site in the region of study.

Analysis of population dynamics: We evaluated the population dynamics at the regional level of each of the

species, by modeling the time series of population growth rates (Lima 2001, Stenseth et al. 2002, Belgrano

et al. 2004B):

Where: it is the intrinsic rate of population growth in the t year, y, and corresponds to the CPUE in the t and

the previous year, respectively.

If this is a feature that gives an account of the relationship between y and , and the delays, then:

Where: It is a linear relationship of dependency between and the delays of abundance.

Due to the short length of the time series (8 years) were evaluated only dynamic of first order (delay for 1

year). In this way, in his simpler version (linear) population dynamics could be modeled on the basis of:

Where:

y are parameter to find the estimate of y

Where: ENV is from any of the oceanographic variables evaluated (TSM, SOI, and UPW).

The choice of the best models was carried out using the Akaike Information Criterion (AIC), where all the

models with a ∆AIC<2 and AIC weighted>0.2 , were considered equally probable. All analysis were carried

out using routines written in the program R (Team 2008).

Results

The populations studied showed different temporal dynamics (Fig 2). The cholga mussel, crab and the hairy

octopus fluctuations showed relatively small populations during the period of analysis. Species such as the

locate and the choro showed sharp increases in the CPUE, whereas the piure, culengue showed drops and

clam population. Other species exhibited fluctuations with more marking, such as the scarlet macaw black,

whose abundance ranged over 2 orders of magnitude in the study. Of the total of 11 models evaluated for

each species (110 in total), only 1-2 were selected in each case (Table 2). The selected models in general

accounted for much of the variance of the population growth (R² average =0.88 [0.46 , 0.99 ], Table 2), so

that the population dynamics of these species was generally well captured by such models (Fig 2). All the

models selected incorporated at the same time effects density-dependent and at least one variable

oceanographic. More than half of the selected models (8 of 15) incorporated delays of first order in the CPUE

and the three variables studied oceanographic. The sign and intensity of the coefficients auto-regressive

associated was variable between species. While the ratio of density-dependence was always negative, the

coefficients associated with the TSM, SOI, and UPW were positive or negative.

Discusión

The results confirm the existence of a marked population regulation in the 10 species of benthic

analysis. This regulation would emerge from the joint action of density-dependent processes and

oceanographic forcing through the feedback of population structure. Traditionally it has been

argued that the oceanographic forcing would have a leading role in fluctuations in abundance of

fisheries (Yañez et al. 2001, Yáñez et al. 2002, Lehodey et al. 2006, Friedemann & Wolff 2008).

However, oceanographic modeling of radiative forcing to the interior structure of the density-

dependent is somewhat recent (Belgrano et al. 2004A, 2004b, Pedraza-Garc ia & Cubillos 2008).

The results give evidence that there are endogenous and exogenous mechanisms that regulate the

overall dynamics of the populations studied. On the one hand, the existence of an endogenous

regulation density-dependent is evident by the negative sign of the coefficients of density-

dependence, α, of all the autoregressive models (Table 2), suggesting that feedback processes of

population of first order would be key in the population dynamics.

. In this sense, a large number of empirical studies validated the importance of larval settlement and

recruitment as a determinant of the structure of populations and marine communities on the coast of

Chile (Lakes et al. 2005, Navarrete et al. 2005). However, it cannot be ruled out a priori in the

existence of delays of higher order ( Pedraza-Garc ia & Cubillos 2008), which would reflect inter-

specific processes (e.g. competition and predation) whose relevance as mechanisms of population

regulation is suggested by empirical evidence (Castile 1999, Gaymer et al. 2004, Vásquez et al.

2006, Gaymer & Himmelman 2008, Navarrete & Manzur 2008). Time Series of greater extension

(> 10 years) would make possible the implementation of these models. On the other hand, these

processes of feedback also would be forced by exogenous factors (i.e. , oceanography) that would

work together by regulating populations, as has been observed in species of marine phytoplankton

(Belgrano et al. 2004A, 2004b).

Experimental studies show that the structure and dynamics of populations and marine communities

in the southeastern coast of the Pacific is heavily governed by stressors of low frequency as El

Niño(Chavez et al. 1999, Thiel et al. 2007) As well as by the larval dispersal patterns generated by

the cells of coastal upwelling (Lakes et al. 2005, Navarrete et al. 2005, Lakes et al. 2008). However

the population effects of these factors track structure density-dependent only have been evident in

the present study. The underlying mechanisms are much less well known, but it is possible that

factors such as SOI indirectly affect recruitment rates of the species, track changes in the intensity

of the transport offshore (Connolly & Roughgarden 1999), or changes in vital rates induced by the

temperature of the sea (Urban 1994, Wolff et al. 2007). However, the direction and intensity of the

coefficients associated with each variable oceanographic was highly variable between species,

which is not strange as there is empirical evidence that indicates that the responses of marine

populations to ENSO events would be species-specific (Díaz & System 1993). The foregoing

suggests that ecological features and/or the history of life could also play a predominant role in

population dynamics. Time series analysis more extensive and the explicit inclusion of the space

(e.g. , analyzing the spatial synchrony of population dynamics) in species with different life

strategies could shed more light on the mechanisms that govern the different population responses

to oceanographic forcing.

The present study demonstrated the feasibility of applying simple autoregressive models for

understanding the role of biological interactions and oceanographic stressors in the regulation of

marine populations. Similar models or more complex (i.e. , non-linear, second-order delays,

explicitly including interactions predator-prey, Pedraza-Garc ia & Cubillos 2008) could be applied

to other stocks for which there are estimates of relative abundance of species. The results obtained

by these simple models could serve as a basis for the development of more sophisticated models

with a view to a sustainable management of marine ecosystems.

Table 1. Main characteristics of the species in this study. Status of holdings according to SubPesca (PE: full exploitation, THE: free access, LAC: free access to administrative restrictions).

Table 1. Main characteristics of the species this study. The exploitation status according SubPesca Chile (PE: fully exploited, THE: open access, LAC: open access with administrative restrictions).

Fig 2. Population Dynamics of 10 marine species commercially extracted by the artisanal fishing

fleet between 1998-2005. The expected values were estimated using the parameters of the

autoregressive model with the best fit for each species (see Table 2).

Fig 2. Population dynamics of 10 marine species exploited by artisanal shellfisheries between

1998-2005. The expected values were estimated using the parameters yielded by the best

autoregressive model for each species (see Table 2).

Agradecimientos

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Tabla 1. Principales características de las especies de este estudio. Estatus de explotación de acuerdo a SubPesca (PE: plena explotación, LA: libre acceso, LAC:

libre acceso con restricciones administrativas).

Table 1. Main characteristics of the species this study. The exploitation status according SubPesca Chile (PE: fully exploited, LA: open access, LAC: open access

with administrative restrictions).

CPUE (kg

viaje-1)

N° de procedencias por año

Taxa Nombre científico Nombre

común

Estatus de

explotació

n

Promedio Promedio Mínimo Máximo

Gastropoda Thais chocolata (Duclos, 1832) Caracol

locate

LAC 390 73 52 90

Echinoidea Loxechinus albus (Molina, 1782) Erizo PE 228 77 28 126Bivalvia Aulacomya atra (Molina, 1782) Cholga PE 208 63 45 71Tunicata Pyura chilensis Molina, 1782 Piure LA 199 65 31 83Bivalvia Gari solida (Gray, 1828) Culengue PE 76 25 5 47Bivalvia Protothaca thaca (Molina, 1782) Almeja PE 70 52 40 69Bivalvia Mytilus chilensis (Hupé 1854) Choro PE 62 16 11 23Decapoda Cancer setosus Fabricius, 1798 Jaiba peluda LAC 54 58 14 74Gastropoda Fissurella latimarginata Lesson,

1831

Lapa negra PE 51 25 7 51

Cephalopoda Octopus mimus Gould, 1852 Pulpo PE 45 130 84 154

Tabla 2. Evaluación de los 11 Modelos auto-regresivos de primer orden de análisis de las dinámicas poblacionales de cada especie (retraso de 1 año en la señal).

Table 2. Summary if the autoregressive models used to analyze the population dynamics of each species. 11 first-order autoregressive models (1 year lag) were

evaluated.

Coeficientes de regresiónEspecie Modelo AIC ∆AIC AICp R² PAlmeja -13,77 0,00 0,519 0,969 0,009 -4,183 -1,194 0,058 0,525

-13,61 0,16 0,479 0,976 0,047 -3,409 -1,241 0,057 0,495 -0,001Caracol

locate

-4,83 0,00 0,316 0,686 0,099 2,944 -0,489 -0,028

-3,91 0,92 0,200 0,731 0,217 4,109 -0,443 -0,031 -0,082Cholga -38,97 0,00 1,000 0,997 0,007 3,309 -1,466 0,038 0,245 0,006Choro -3,88 0,00 0,868 0,953 0,091 15,036 -1,110 -0,132 -0,588 -0,007Culengue 10,09 0,00 0,522 0,824 0,321 -14,928 -2,099 0,182 1,316 0,026Erizo -5,36 0,00 0,379 0,458 0,554 14,486 -0,460 -0,675 -0,005

-4,66 0,69 0,268 0,451 0,302 14,425 -0,631 -0,627Jaiba peluda -12,34 0,00 0,605 0,976 0,047 4,428 -1,130 -0,017 0,004

-10,35 1,99 0,223 0,953 0,093 4,563 -1,137 -0,018 -0,006 0,004Lapa negra 3,65 0,00 0,988 0,979 0,043 28,828 -2,315 -0,077 -1,249 0,052Piure -25,92 0,00 0,998 0,994 0,011 -2,567 -0,829 0,057 0,388 0,004Pulpo -14,05 0,00 0,577 0,968 0,062 0,463 -1,776 0,010 0,362 -0,005

-13,24 0,81 0,385 0,953 0,017 1,542 -1,565 0,256 -0,005

Fig. 1. Mapa del área de estudio en el norte grande de Chile, mostrando la localización

geográfica de las 231 áreas de pesca en el área de estudio (cuadrados claros).

Fig. 1. Map of the study area at the northern Chile region, showing the geographic location of

the 231 fishing localities used in this study (empty squares).

Fig. 2. Dinámica poblacional de 10 especies marinas extraídas comercialmente por la flota

pesquera artesanal entre 1998-2005. Los valores esperados se estimaron usando los

parámetros del modelo autoregresivo con el mejor ajuste para cada especie (ver Tabla 2).

Fig. 2. Population dynamics of 10 marine species exploited by artisanal shellfisheries between

1998-2005. The expected values were estimated using the parameters yielded by the best

autoregressive model for each species (see Table 2).