The Effects of The Environment on The Black Sea Anchovy - Signals on The Growth

Akkuş, Gizem
The Black Sea anchovy, Engraulis encrasicolus (Linnaeus, 1758), is a fast-growing, short-lived small pelagic fish species. This fish stands out economically as it alone accounts for more than 60% of the total landings in the Black Sea. Being one of the most targeted species in commercial fisheries of the Black Sea, the sustainable exploitation of anchovy is aimed. Accordingly, it has been managed by the recommendations given as a result of age-based scientific stock assessment models in recent years. For these age-based models, age data representing the population is of great importance. The accuracy of age determinations and their consistency across countries in the region have always been a source of concern, from the earliest assessment studies to the present day. In addition, parameterizing the growth of the fish in a way that reflects the stock’s overall health and resilience is also an important factor in order to deliver more reliable results for the models. Indeed, these models are already operating by assuming that these data are completely correct and reflect the stock. Therefore, estimating these age and growth, from which the biological life history of the fish is derived, is consistent and the margin of error is minimized, also makes it possible to reduce the uncertainties in the model. In addition, the 'Unit Stock' concept is another presumption that assessment models assume to be accurate. For this reason, a correct definition of the stock to be managed brings this assumption closer to the truth. The ongoing assumption in the management of Black Sea anchovy is to accept that the Black Sea anchovy and the Azov anchovy are separate stocks and they never mix. However, considering the anchovy in the Black Sea, the discussions regarding the separation of Black Sea and Azov anchovy stocks, which have been going on since the early 20th century, have not yet reached a clear conclusion. Nevertheless, according to the literature, it can be said that there are at least two separate stocks in the Black Sea region. However, while recent genetic studies emphasize that there is no genetic difference between these two groups, they attribute their differentiation to the effects of environmental factors. As a result, regional stock differences have come to the fore. But, particularly when anthropogenic and climate-induced changes are taken into account, regional differentiation has become more difficult with the disappearance of environmental barriers between them. On the other hand, the information that these two groups grow differently is underlined in the literature. Yet, a comprehensive study on this subject is not available. The ultimate aim of this thesis is to find a way to obtain accurate information reflecting the stock by minimizing the uncertainties arising from the estimation of parameters such as age and growth to contribute to the sustainable management of anchovy in the Black Sea. In addition, the method of separating the different stocks mentioned in the Black Sea based on the growth information has been questioned. The otoliths, an important tool in fisheries science, are used to understand fish age, growth rate, fish response to environmental conditions, and population dynamics in general. This information obtained is of great importance for sustainable fisheries management. In this study, sagittal otoliths from 3150 anchovy specimens collected during scientific surveys conducted in 2012, 2013, 2014, 2015, 2016, 2018, and 2020 covering the entire Turkish Exclusive Economic Zone were used to get both age and growth information. It was revealed that there is a statistically significant relationship between the otolith radius and the total fish length (R2=0.92; p-value < .001). Opaque and hyaline rings, traces of fast and slow growing periods, respectively, were used for age determination. The radius of these increment rings from the otolith center were measured. From here, the growths for each age were calculated. A total of 3960 incremental radii data were measured for this aim. As a result of this thesis, two separate outcomes were produced that will directly affect the model results. The first is the "Black Sea anchovy age reading protocol". This protocol is currently used throughout the Black Sea region. In this way, standard and comparable age-length keys can be produced. Another is the more accurate growth parameters that reflect the stock. So far, growth parameters for anchovy in the Black Sea have been obtained using the mean size at age method. These were generally estimated using samples collected at different times of the year. This method causes a shift in the age-corresponding length information depending on the time of the year that the sample is collected, thus causing errors in the estimations. In addition, t0 values that are too small to be true are estimated while constructing the growth curve. This reduces the reliability of the growth prediction. In order to avoid all these sources of error, in this study the growth parameters were estimated with the von Bertalanffy approach by using the growth increment radii-at-age as a proxy for the exact total length of the fish-at-age. Utilizing this method, Linf=12.9 cm, K=0.765, and t0= -0.134 were estimated for anchovies in the southern Black Sea region. This proposed method makes it possible to obtain growth information from otoliths that have already been collected to get age information, as well as to predict retrospective growth parameters. In the last decades, very significant ecological changes have occurred in the Black Sea, and these changes have had a significant impact on many living beings. Anchovy is known to be severely affected by these changes. The existence of anchovy subgroups in the Black Sea region, which makes the definition of "unit stock" difficult, is also discussed as a part of these changes. In this study, the studies that have been done to date, which are still available, are compiled and presented in Chapter 2. According to the conclusion drawn from this review, anchovy groups in the Black Sea were defined as environmental ecotypes. In this thesis, the existence of different groups was investigated by their growth rates in the first year of their life, which corresponds to ~72% of their total asymptotic growth, with the knowledge that different forms grow differently and with the reference that growth is a response to the environment of the fish. The first-year growth information was obtained by measuring the distance from the center of the otolith to the edge of the increment corresponding to the first age. Accordingly, two different anchovy groups labeled as “Group 1” and “Group 2” that grow differently (ɸ1=2.05, ɸ2=2.29) have been detected in the southern Black Sea region (likelihood ratio test; chi-square statistics: p-value < .001, df=3). So, estimated growth parameters according to the method proposed in Chapter 3 for Group 1 and Group 2, respectively: Linf (cm) = 12.4 and 12.1; K(1/year) = 0.726 and 1.347; t0 (year)=-0.145 and -0.079 were found. Finally, the hypothesis was tested that if disparate growth patterns among groups arise due to variations in the seawater properties of their respective spawning environments, then, later, they will instinctively prefer water bodies similar to those water. This propensity persists even if the intermixing occurs after experiencing a winter. Moreover, it was also tested that these different water masses will eventually influence the growth of the young-of-year. For this purpose, the temperature, salinity, and fluorescence values were measured at a total of 363 different stations in the years 2013, 2015 and 2018 as part of the summer season (July-August). Subsequently, the seawaters of 75 different stations where anchovy samples were caught were characterized. Accordingly, separate water masses with different characteristics and covering different regions were identified for each year. The otoliths collected from the summer season were grouped, and two different groups were identified that grew differently according to their growth in the first years of their lives. In addition, the otolith radii of the young-of-year, who were only one and a half months old at the time of sampling, corresponding to the estimated birth date of June 1, were used as growth markers. According to the results of the study, contrary to expectations, it was found that adult individuals who experienced a winter were intermixed in the basin and the detected different groups did not prefer a particular water mass (ANOVA2013; F (1, 75) = [2.0812], p-value = 0.1533; ANOVA2015; F (1, 112) = [3.374], p-value = 0.06888.; ANOVA2018; F (2, 84) = [0.5873], p-value = 0.5581). However, the results show that even though the water mass characteristics they are in do not matter for adults, the situation is different for the young-of-years. A statistically significant relationship was found between the growth of the offspring of the sampled year and the water masses in which they were caught (ANOVA; F (2, 515) = [3.863], p-value = 0.022). In addition, when examining the average growth of these young-of-years, in 2015, which is characterized by the high amount of fluorescence, the highest mean growth was noted (M=0.818 mm, SD=0.184). Yet, the lowest average young growth was recorded in 2018, which was the hottest among sampling years (M = 0.767 mm, SD = 0.159). As a result, it will be possible to track the cohorts within the model with the age-length key for each sampling year generated by the age reading protocol prepared within the scope of this thesis study, which was prepared with a stock assessment perspective. Thus, a more accurate model result reflecting the stock will be achieved. Growth estimates obtained using the otolith method can be used to estimate fishery-dependent and independent parameters with minimal uncertainty. This study identified different growing anchovy groups from otolith samples collected as part of fishery-independent scientific research surveys. The different growing group compositions in the catch can also be determined using the same method, that is, with the first-year growth information obtained from otoliths collected for age reading. In this way, the ratios of different stocks in the catch can be found and the stocks may be managed accordingly. As a result of this study, it is recommended to reconsider the unit stock assumptions made and the size-selective fishery regulation for the Black Sea anchovy. It is further recommended to take into consideration that juvenile anchovy populations, particularly those in their early stages of development, exhibit strong sensitivity to environmental factors. This consideration bears significance in the establishment of stock management plans. Furthermore, it is anticipated that these results will provide a representative framework for future studies examining the effects of climate change on anchovy growth.
Citation Formats
G. Akkuş, “The Effects of The Environment on The Black Sea Anchovy - Signals on The Growth,” Ph.D. - Doctoral Program, Middle East Technical University, 2023.