Life-history and behaviour of "Labrus bergylta" in Galicia
UNIVERSAL IDENTIFIER: http://hdl.handle.net/11093/98
UNESCO SUBJECT: 5102.08 Pesca ; 2401.06 Ecología Animal ; 2401.02 Comportamiento Animal ; 3105.04 Protección de Los Peces
DOCUMENT TYPE: doctoralThesis
Life-history and behaviour determine the capacity of fish populations to withstand human impacts, namely fishing. Artisanal fisheries are often viewed as having little impact on marine ecosystems. However, a number of studies worldwide have provided evidence of how this activity may deplete target populations and trigger cascading effects on local ecosystems. In this PhD thesis I investigated the life-history traits and behavioural patterns of a highly exploited fish species in Galicia, the ballan wrasse Labrus bergylta. I aimed to understand how the specific reproductive, growth, bioenergetic and behavioural patterns determine its vulnerability to fishing and other human impacts. Since the study species display two main body colour patterns, I maintain a comparative analysis of the results between both morphotypes all over the PhD thesis. Firstly, in chapter 1 I give a general overview of the implications of life-history and behaviour of fishes for fisheries management and vulnerability. I also present past research results on Labrus bergylta, and provide baseline information on the situation of the artisanal fishery in Galicia and the main characteristics of the study area. I close this section with the motivation and the objectives of this PhD thesis. Chapter 2 is aimed at describing the age-based demographic parameters of the species, based on otolith readings, biological sampling and catch data. I validated the annual periodicity of annuli deposition by oxytetracycline injection of captive fish. Labrus bergylta displayed a moderate longevity within the context of the Labridae family, with maximum ages of 22 yr. A strong relationship was found between the otolith weight and fish age and between otolith length and fish length, but those relationships differed between colour patterns. Males were significantly larger and older than females, whereas spotted individuals were significantly larger, but not older, than plain individuals. Males and spotted individuals attained a larger mean asymptotic size (40.36 and 49.32 cm respectively) than females (37.51 cm) and plain (36.60 cm) individuals, respectively, but converged to it more slowly. Estimates of mortality based on catch curves from two independent datasets provided a global mortality of 0.38 yr-1 and 0.40 yr-, respectively. However, significant higher mortality was obtained for females (0.51 yr-) as compared to males (0.32 yr-) and for plain individuals (0.42 yr-) as compared to the spotted individuals (0.28 yr-). Chapters 3 and 4 focused on the reproductive traits of the species. Labrus bergylta spawned in winter in Galicia, between January and April. Female oocyte development type was group synchronous, whereas fecundity type was determinate. Maturity of 50% of the population was attained at approximately 23.5 cm and 2.4 yr. Plain individuals attained the length and age at 50% sex change earlier (36 cm and 7.4 yr) than spotted individuals (47.2 cm and 11.8 yr). In addition we found that individuals with gonads infected by encysted trematode metacercariae were associated with an earlier timing of sex change (30.6 cm and 2.9 yr) as compared to uninfected individuals (36.5cm and 7.9 yr), what represents one of the few examples of a possible parasitic influence on the sex allocation of its host. Energy allocation is an important component of life history variation since it determines the tradeoff between growth and reproduction. In chapter 4 we investigated the energy allocation patterns and reproductive investment of Labrus bergylta, as an example of a protogynous hermaphrodite fish with parental care. Energy content and gut weight (as a proxy of feeding intensity) were modeled in relation to the reproductive phase of the individuals. A decrease in the stored energy (mesenteric fat and liver energy content) from prespawning to spawning paralleled the increase in the gonad energy content in the same period. Mesenteric fat accounted for most of the stored energy both in males and females. The predicted reduction in stored energy over the reproductive cycle was higher than the energy required to develop the ovaries for the full range of female sizes analysed, suggesting a capital breeding strategy. Males stored less energy over a season and invested fewer resources in gamete production. Feeding intensity was reduced during spawning, but this reduction was more intense in males than in females. Reproductive investment (both fecundity and energy required to produce the gonads) were higher in plain than in spotted females, what is in agreement with the different growth patterns described for the species. In addition effective fisheries management needs to consider spatial behaviour in addition to more traditional aspects of population dynamics. Acoustic telemetry has been extensively used to provide information on fish movements over different temporal and spatial scales. In chapter 5, we used a fixed-receiver array to examine the movement patterns of Labrus bergylta for a period of 366 days in the Galician Atlantic Islands Maritime-Terrestrial National Park. Fish were present within the study area more than 89% of time. The estimated size of individual home ranges, i.e. the area where fish spent most of their time, was small. The total minimum convex polygons area based on all the estimated positions was 0.177 ± 0.071 km2, whereas the home range size estimated using a 95% kernel distribution of the estimated positions was 0.097 ± 0.032 km2. The core area (50% kernel) was 0.018 ± 0.006 km2. L. bergylta exhibited different patterns of movement in the day versus the night. All fish travelled longer distances during the daytime and 72% of the fish displayed a larger home range during the day versus night in a daily basis. The linearity index was less than 0.002 for all fish suggesting random movements but within a relatively small area, and the volume of intersection index between consecutive daily home ranges was 0.75 ± 0.13, suggesting high site fidelity. Chapter 6 described the seasonal patterns of behaviour, their potential drivers and consequences. We focused on the home range size as a proxy of the exploratory behaviour, and the distance travelled as a proxy of fish activity. Distance travelled revealed a clear seasonal pattern, but home range size did not. Feeding to maintain temperature-related metabolism seemed to be the main driver of the activity intensity, rather than spawning. The pattern of variation of the catch rates along the year paralleled the seasonal trend of activity. Therefore, fish activity seemed to determine the catchability of L. bergylta. This result highlights the importance of considering fish behaviour when interpreting catch per unit effort data as index o abundance. Finally, in chapter 7 I discuss the implications of the results from a management perspective. I highlight the importance of considering intraspecific variation in fish populations and identify the weaknesses of the present management tools, which are based primarily on a minimum catch size, due the complex life history of the species. I also propose alternative regulations to effective protect the species. Marine protected areas are suggested as one of the main alternatives to effectively protect the whole population structure. To finish, I highlight some interesting future research lines that stem directly from this PhD thesis.
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