Methods for integrated use of fisheries research survey information in understanding marine fish population ecology and better management advice : improving methods for evaluation of research survey information under consideration of survey fish detection and catch efficiency

Summary

The thesis developed and improved methods for the integrated analysis of different types of fishery independent research surveys (trawl surveys, acoustic surveys, hydrographical surveys, and gillnet surveys) to study the distribution, density, abundance, migration and biological population dynamic parameters of marine fish species. The topics in the thesis addressed different combinations of trawl, hydro-acoustic, gillnet, and hydrographical data and application of different survey data analysis methods under consideration of factors influencing the survey catch and detection efficiency. Each topic was investigated in one of more case studies.

One thesis topic has been to provide more precise estimates of fish distribution and density patterns from survey data (Chapter 2). The 1st case study applied advanced statistical methods to Baltic trawl data and hydro-acoustic survey data in combination with survey sampled hydrographical data to estimate distri­bution and density patterns of juvenile 0-group Baltic cod. These patterns were largely unknown. In the 2nd case study new methodology was developed for analyzing trawl research survey data for Baltic cod and whiting including the corre­la­tion in distribution and density according to space, time, size, and species. The more precise density estimates improve the knowledge of the stock-recruitment relationship of Baltic cod and can improve the Baltic multi-species stock assessment. Furthermore, it will enable more precise marine management and spatial planning involving fish stocks and fisheries in the Baltic Sea. In context of Baltic cod stock assessment, the 3rd case study developed a new method for inter-calibration of trawl survey CPUE data by fish size group exemplified by Baltic cod (and flounder) where the concept of disturbance by one trawl haul in relation to the next have been developed and quantified when calibrating new research survey trawl gears with the former ones. These results have been based on introduction of a new international ICES BITS trawl research survey design.

A second topic was to improve and develop hydroacoustic research survey methods for more precise detection and discrimination of fish species according to fish size and orientation in the water (Chapter 3). Here, the 4th case study focused on more precise acoustic target strength estimation of juvenile cod, while the 5th case study has focused on acoustic discrimination of juvenile gadoid fish in particular juvenile Baltic cod. This enables more efficient research survey estimation of juvenile cod (gadoid) density patterns to be used in stock recruitment estimates and stock assessment.

The third topic was to estimate more precisely fish mortality, maturity, and growth parameters for small forage fish species using research survey information (Chapter 4). Associated to this, the 5th case study analysed these population dynamic parameters using trawl survey data taking into account spatial variation. This study provided more precise estimates and deeper understanding of Norway pout mortality, maturity, and growth dynamics. The more precise population dynamic parameters have been implemented in and improved the North Sea Norway pout stock assessment, management advice, management, and long term management plan evaluations.

The fourth topic was to develop methodology to integrate hydroacoustic, gillnet, and hydro­gra­phi­cal research survey data to investigate pelagic fish migration patterns (Chapter 5). The methods were applied in the 6th case study to evaluate Western Baltic herring feeding and spawning migra­tion based on distribution and density estimates in a narrow over-wintering area of the stock. The more precise information on migration patterns gives better possibility for acoustic monitoring of the full stock abundance in different areas and seasons og the year to be used in stock assessment and marine spatial planning. Also, it increases knowledge on biological interactions and mixing with other stocks and species.  

In the final synthesis Chapter 6, the thesis reviews relevant analysis methods of research survey data and underlying data distributions, survey design and stratification, trawl survey inter-calibration and standardization, as well as estimation procedures and data processing methods in context of the obtained results and methods developed in the thesis. This is done with focus on survey precision and uncertainty (bias, sources of errors) for trawl and acoustic surveys and factors affecting it.