Analysis of trophic interactions of large oceanic predators as an indicator of the stability of marine ecosystems
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Fecha
2019
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Idioma
es
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Universidad Andrés Bello
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Licencia CC
Licencia CC
Resumen
El aumento en la actividad pesquera ha provocado un importante aumento en las poblaciones
de especies marinas que se encuentran sobreexplotadas a niveles no sostenibles. Esto ha llevado
a una preocupante pérdida de biodiversidad lo cual puede favorecer el colapso de la actividad
pesquera, con importantes consecuencias ecosistémicas, económicas y sociales. Un manejo
pesquero adecuado, que asegure la sustentabilidad del recurso pesquero, requiere conocer el
estado del ecosistema. Una forma de conocer el estado de un ecosistema explotado es por medio
del análisis de su estabilidad, estado en el cual el ecosistema tiene las condiciones adecuadas
para regresar a un estado inicial después de una perturbación. Un elemento fundamental para
que el ecosistema tenga esta capacidad y sea estable, es la diversidad de sus componentes. De
esta forma, la asimetría en los nichos entre y dentro de las especies permite un mayor número
de opciones que ayudan a las comunidades a responder y recuperarse de las perturbaciones. Por
este motivo, la presencia de nichos muy similares (o simétricos) sugieren que un ecosistema está
siendo impactado a un nivel donde su estabilidad puede verse comprometida. En el caso de los
depredadores de alto nivel trófico, su plasticidad trófica les permite adaptarse a los cambios en
las condiciones del ambiente. En condiciones normales, sus nichos están segregados, con una
baja superposición y alta asimetría, sin embargo, frente a perturbaciones, como la reducción en
los recursos o de otro depredador, su plasticidad permite modificar sus nichos, favoreciendo la
superposición y simetría de éstos, afectando la estabilidad del ecosistema.
En Chile una de las pesquerías más importantes es la del pez espada (Xiphias gladius) donde,
de forma secundaria, son capturados tiburones (Isurus oxyrinchus y Prionace glauda) y como
fauna acompañante atunes y escolares. Esta actividad, de más de 40 años, se ha caracterizado
por iniciar como una pesca sin control hasta que fue regulada, manteniendo una tendencia
creciente. Si bien el análisis de las poblaciones de estos depredadores sugiere una estabilidad
del stock entre los años 2009 al 2017, la población del azulejo tuvo una reducción en su
rendimiento, lo que se suma a la alteración de la productividad del ecosistema pelágico
evidenciado a través de la disminución del nivel trófico. Esta situación supone un gran riesgo
para la actividad pesquera nacional, por la posible pérdida de elementos clave del ecosistema,
promoviendo la reducción en su estabilidad.
Para estudiar la estabilidad del ecosistema oceánico centro-norte de Chile, con datos obtenidos
entre los años 2012 y 2017, se analizaron, por un lado, los grados de superposición de los nichos
isotópicos de nueve depredadores de alto nivel tróficos que co-habitan, y por otro lado, se
estudiaron las superposiciones entre las dietas, así como los cambios en la posición trófica de
los tiburones azulejo y mako, y del pez espada. Con esta información, se analizó la correlación
entre los valores de superposición (de nichos isotópicos y dietas) con factores pesqueros
(esfuerzo, rendimiento y desembarcos) para evaluar el impacto de esta actividad sobre la
estabilidad del ecosistema. De forma complementaria, debido a que la pesquería puede verse
afectada por variaciones en el ambiente, se incorporó factores oceanográficos (SST y Clorofilaa) para analizar su posible efecto. En el estudio de los nichos isotópicos, los resultados sugirieron
que las interacciones entre los depredadores fueron variables en el tiempo, con promedios y
proporciones de superposición entre las Especies y Grupos de Depredadores para cada año que
no permitieron indicar de forma robusta que existió una menor estabilidad en el ecosistema
evaluado. Sin embargo, el registro de superposiciones significativas todos los años y ausencia
de diferencias significativas en los valores isotópicos en algunas de las interacciones, sugiere
posibles alteraciones en la estabilidad del ecosistema. En el estudio de las dietas, los análisis de
superposición a nivel de macro-grupo de presas con SCA y a nivel de grupo de presas y macrogrupo de presas con SIA entregan indicios que la estabilidad del ecosistema puede estar siendo
afectada negativamente. De forma similar, la tendencia decreciente de la posición trófica,
especialmente para el azulejo y el mako, sugieren un impacto negativo sobre el ecosistema en
sus tramas tróficas. Finalmente, en el estudio de la correlación, los resultados no permitieron,
de forma robusta, vincular el grado de superposición entre depredadores (nicho isotópico o
dieta) con alguno de los factores pesqueros y oceanográficos considerados, de todos modos, las
observaciones permiten sugerir que los factores oceanográficos, por sobre los factores
pesqueros, tuvieron una mayor influencia en los proxy de estabilidad del ecosistema aquí
estudiados. Sin embargo, las escasas correlaciones detectadas no permiten dar resultados
concluyentes.
Los resultados sugieren que en el ecosistema oceánico frente a las costas centro-norte de Chile
están ocurriendo eventos que causan el incremento en la superposición de los depredadores,
disminuyendo la asimetría y desfavoreciendo la capacidad de resiliencia del ecosistema frente
a perturbaciones. Sin embargo, este trabajo no puedo correlacionar de forma robusta la actividad
pesquera con el incremento en las superposiciones. De todos modos, los datos otorgados por
esta investigación parecieran ser la antesala de futuras consecuencias mayores sobre el
ecosistema, en especial si las medidas de manejo pesquero permanecen iguales. Por lo que no
deben ser desconocidos y se requiere de un seguimiento del estudio, que permita analizar
cambios futuros, para poder generar proyecciones que contribuyan a la implementación de
planes de gestión adaptativos en función de las necesidades del ecosistema, para de esta forma
asegurar, la sustentabilidad de la actividad, prolongando los beneficios para las personas.
The increase in fishing activity has led to a significant increase in the populations of marine species that are overexploited at unsustainable levels. This has led to a worrying loss of biodiversity which can lead to the collapse of fisheries, with significant ecosystem, economic and social consequences. Adequate fishing management, which ensures the sustainability of the fishing resource, requires knowing the state of the ecosystem. One way to know the state of an exploited ecosystem is by analyzing its stability, a state in which the ecosystem has the right conditions to return to an initial state after a disturbance. A fundamental element for the ecosystem to have this capacity and be stable is the diversity of its components. Thus, asymmetry in niches between and within species allows for a greater number of options that help communities respond to and recover from disturbances. For this reason, the presence of very similar (or symmetrical) niches suggests that an ecosystem is being impacted at a level where its stability may be compromised. In the case of high trophic predators, their trophic plasticity allows them to adapt to changing environmental conditions. Under normal conditions, its niches are segregated, with a low superposition and high asymmetry, however, in the face of disturbances, such as the reduction in resources or another predator, its plasticity allows its niches to be modified, favoring their overlapping and symmetry, affecting the stability of the ecosystem. In Chile, one of the most important fisheries is swordfish (Xiphias gladius), where sharks (Isurus oxyrinchus and Prionace glauca) are secondarily caught, and tuna and oilfish are accompanying fauna. This activity, more than 40 years old, has been characterized by starting as a fishing without control until it was regulated, maintaining a growing trend. Although the analysis of the populations of these predators suggests a stability of the stock between 2009 and 2017, the blue shark population had a reduction in its yield, which adds to the alteration in productivity of the pelagic ecosystem evidenced through the decrease in the trophic level. This situation poses a great risk to the national fishing activity, due to the possible loss of key elements of the ecosystem, promoting a reduction in its stability. In order to study the stability of the north-central oceanic ecosystem of Chile, the isotopic niches of nine high trophic predators were analyzed, and on the other hand, the overlap between the diets and the changes in the trophic position of the blue shark, mako shark and swordfish were studied. With this information, the correlation between overlapping values (of isotopic niches and diets) and fishing factors (intensity, yield and landings) was analyzed to evaluate the impact of this activity on ecosystem stability. Complementarily, because the fishery may be affected by variations in the environment, oceanographic factors (SST and Chlorophyll-a) were incorporated to analyze their possible effect. In the study of isotopic niches, the results suggested that interactions among predators were variable over time, with averages and proportions of overlap between Species and Predator Groups for each year that did not allow to indicate in a robust way that there was less stability in the evaluated ecosystem. However, the record of significant overlaps every year and the absence of significant differences in isotopic values in some of the interactions, suggests possible alterations in the stability of the ecosystem. In the study of diets, overlap analyses at the prey macro-group level with SCA and at the prey group level and prey macro-group with SIA provide clues that ecosystem stability may be negatively affected. Similarly, the decreasing trend of trophic position, especially for blue and mako sharks, suggest a negative impact on the ecosystem’s trophic web. Finally, in the study of correlation, the results did not allow, in a robust way, to link the degree of overlap between predators (isotopic niche or diet) with some of the fishing and oceanographic factors considered. However, the observations suggest that possibly oceanographic factors, more than fishing factors, had a greater influence on the proxy of ecosystem stability, however, the few correlations detected do not allow conclusive results. All the results provide evidence suggesting that in the oceanic ecosystem off the north-central coast of Chile, events are occurring that cause an increase in the overlapping of predators, decreasing asymmetry and disadvantaging the resilience of the ecosystem to disturbances. However, this work cannot correlate fishing activity in a robust way with the increase in overlaps. In any case, the data provided by this research seem to be the prelude to future major consequences on the ecosystem, especially if fisheries management measures remain the same. Therefore, they should not be unknown and a follow-up of the study is required to analyze future changes in order to generate projections that contribute to the implementation of adaptive management plans based on the needs of the ecosystem, thus ensuring the sustainability of the activity, prolonging the benefits for people.
The increase in fishing activity has led to a significant increase in the populations of marine species that are overexploited at unsustainable levels. This has led to a worrying loss of biodiversity which can lead to the collapse of fisheries, with significant ecosystem, economic and social consequences. Adequate fishing management, which ensures the sustainability of the fishing resource, requires knowing the state of the ecosystem. One way to know the state of an exploited ecosystem is by analyzing its stability, a state in which the ecosystem has the right conditions to return to an initial state after a disturbance. A fundamental element for the ecosystem to have this capacity and be stable is the diversity of its components. Thus, asymmetry in niches between and within species allows for a greater number of options that help communities respond to and recover from disturbances. For this reason, the presence of very similar (or symmetrical) niches suggests that an ecosystem is being impacted at a level where its stability may be compromised. In the case of high trophic predators, their trophic plasticity allows them to adapt to changing environmental conditions. Under normal conditions, its niches are segregated, with a low superposition and high asymmetry, however, in the face of disturbances, such as the reduction in resources or another predator, its plasticity allows its niches to be modified, favoring their overlapping and symmetry, affecting the stability of the ecosystem. In Chile, one of the most important fisheries is swordfish (Xiphias gladius), where sharks (Isurus oxyrinchus and Prionace glauca) are secondarily caught, and tuna and oilfish are accompanying fauna. This activity, more than 40 years old, has been characterized by starting as a fishing without control until it was regulated, maintaining a growing trend. Although the analysis of the populations of these predators suggests a stability of the stock between 2009 and 2017, the blue shark population had a reduction in its yield, which adds to the alteration in productivity of the pelagic ecosystem evidenced through the decrease in the trophic level. This situation poses a great risk to the national fishing activity, due to the possible loss of key elements of the ecosystem, promoting a reduction in its stability. In order to study the stability of the north-central oceanic ecosystem of Chile, the isotopic niches of nine high trophic predators were analyzed, and on the other hand, the overlap between the diets and the changes in the trophic position of the blue shark, mako shark and swordfish were studied. With this information, the correlation between overlapping values (of isotopic niches and diets) and fishing factors (intensity, yield and landings) was analyzed to evaluate the impact of this activity on ecosystem stability. Complementarily, because the fishery may be affected by variations in the environment, oceanographic factors (SST and Chlorophyll-a) were incorporated to analyze their possible effect. In the study of isotopic niches, the results suggested that interactions among predators were variable over time, with averages and proportions of overlap between Species and Predator Groups for each year that did not allow to indicate in a robust way that there was less stability in the evaluated ecosystem. However, the record of significant overlaps every year and the absence of significant differences in isotopic values in some of the interactions, suggests possible alterations in the stability of the ecosystem. In the study of diets, overlap analyses at the prey macro-group level with SCA and at the prey group level and prey macro-group with SIA provide clues that ecosystem stability may be negatively affected. Similarly, the decreasing trend of trophic position, especially for blue and mako sharks, suggest a negative impact on the ecosystem’s trophic web. Finally, in the study of correlation, the results did not allow, in a robust way, to link the degree of overlap between predators (isotopic niche or diet) with some of the fishing and oceanographic factors considered. However, the observations suggest that possibly oceanographic factors, more than fishing factors, had a greater influence on the proxy of ecosystem stability, however, the few correlations detected do not allow conclusive results. All the results provide evidence suggesting that in the oceanic ecosystem off the north-central coast of Chile, events are occurring that cause an increase in the overlapping of predators, decreasing asymmetry and disadvantaging the resilience of the ecosystem to disturbances. However, this work cannot correlate fishing activity in a robust way with the increase in overlaps. In any case, the data provided by this research seem to be the prelude to future major consequences on the ecosystem, especially if fisheries management measures remain the same. Therefore, they should not be unknown and a follow-up of the study is required to analyze future changes in order to generate projections that contribute to the implementation of adaptive management plans based on the needs of the ecosystem, thus ensuring the sustainability of the activity, prolonging the benefits for people.
Notas
Tesis (Doctor en Medicina de la Conservación)
Palabras clave
Ecología Marina, Biodiversidad Marina, Animales Depredadores Marinos, Investigaciones