Sea

PhD defence about the environmental effects on results from trawl surveys

Thursday 16 Nov 17

Contact

Contact

Kai Wieland
Head of section
DTU Aqua
+45 35 88 32 76

Time & Place

Time
Tuesday, 21 November 2017, 1 p.m.

Place
DTU Aqua, North Sea Science Park, Willemoesvej 2, 9850 Hirtshals

 

The thesis

A copy of the thesis is available for reading at DTU Aqua. Contact PhD Secretary Rikke Hansen, rikh@aqua.dtu.dk

PhD Student Paulus Inekela Kainge has investigated the effects of variations in e.g. oxygen, temperature and wind on collection of data on hake stocks in Namibia. On 21 November he will defend his thesis

Studies on demersal fish species have shown that variability in environmental conditions, e.g. oxygen, temperature, wind and time of day, during trawling may result in differences in the catching efficiency of the trawl gear, which may cause differences in abundance estimations of stocks. 

As a PhD student at DTU Aqua and the National Marine Information and Research Centre in Namibia, Paulus Inekela Kainge has investigated the effects of these environmental conditions on trawl survey abundance indices for the Cape hakes, Merluccius capensis and Merluccius paradoxus, through an analysis of existing survey data catch, in order to gain a better understanding of the behavioral processes involved. This is crucial for improving the reliability of the hake stock assessment, and it is directly linked to the validation or modification of the current assessment practices.

Results indicate that time of day has an effect on survey catch rates, mostly for M. capensis, where lower catch rates were obtained during the night, in shallower waters. In addition, the most important oceanographic covariates affecting catch rates are bottom oxygen, bottom depth, geographical position and bottom temperature and this is an indication that the survey size structure may have been affected by the behavior of both species towards environmental conditions. 

About the defence

Paulus Inekela Kainge will defend his PhD thesis “Environmental effects on the availability of shallow and deep water hake to the demersal trawl survey in Namibian waters” on Tuesday, 21 November 2017. 

The defence will take place at 1 p.m.  at DTU Aqua, North Sea Science Park, Willemoesvej 2, 9850 Hirtshals.

Supervisors

  • Main supervisor: Senior Researcher Kai Wieland, DTU Aqua
  • Co supervisors: Senior Researcher Niels Gerner Andersen, DTU Aqua and Hashali Hamukuaya, International Benguela Current Comission, Namibia

Examiners

  • Professor J. Rasmus Nielsen, DTU Aqua
  • Associate Professor Peter Grønkjær, University of Aarhus
  • Senior Scientist Hilkaa Ndjaula, University of Namibia

Chairperson at the defence

  • Senior Researcher Ludvig Ahm Krag

Summary of the thesis

Studies on several demersal fish species have shown that variability in environmental conditions during trawling may result in differences in the catching efficiency of the trawl gear, which may cause differences in abundance estimations of stocks. This is even complicated in the case of the shallow and deep water hake, Merluccius capensis and Merluccius paradoxus, which are known to perform diurnal vertical migrations possibly for spawning or in search of food.

Paulus Inekela Kainge's PhD aims at investigating the effects of bottom environmental conditions on trawl survey abundance indices through an analysis of existing survey CPUE data, in order to gain a better understanding of the behavioral processes involved. This is crucial for improving the reliability of the hake stock assessment, and it is directly linked to the validation or modification of the current assessment practices. 

This was achieved through a literature review and an analysis of existing data from the Namibian annual routine monitoring survey in respect to time of day and environmental effects on bottom trawl catches of hake as well as dedicated new field sampling which also included a study on feeding behaviour. 

Results indicate that time of day has an effect on survey catch rates, mostly for M. capensis, where lower catch rates were obtained during the night, in shallower waters. In addition, the most important oceanographic covariates affecting catch rates are bottom oxygen, bottom depth, geographical position and bottom temperature and this is an indication that the survey size structure may have been affected by the behavior of both species towards environmental conditions. 

Both hake species fed more on semi-demersal and demersal components of the prey field, as well as on pelagic species like krill and myctophids. A hake-on-hake predation was also observed, with both hake species occurring as prey in the stomachs of M. capensis while only M. paradoxus occurred in the stomachs of M. paradoxus

Results of the different papers are synthesized in relation to diagnosing environmental effects on survey catchability and then suggestions for time series adjustments are provided.