Effects of environmental variables on survey catch rates and distribution by size of shallow- and deep-water Cape hakes, Merluccius capensis and Merluccius paradoxus off Namibia

Paulus Inekela Kainge, A. K. van der Plas, C. H. Bartholomae, Kai Wieland

Research output: Contribution to journalJournal articleResearchpeer-review

Abstract

In order to study the effects of temperature, oxygen, salinity and time of day on
survey trawl catches, we modeled observed catches of juvenile, small, medium and large hakes per station as functions of zenith angle of the sun, geographical position, year, temperature, salinity, oxygen and depth. We used data from summer demersal surveys conducted during the period 2002–2015, together with a computation of the corresponding light level data from which the solar zenith angles were obtained, and fitted the generalized additive models to these data. Based on best model results, important covariates were oxygen, depth, geographical position and temperature. The best models explained 70%, 69%, 57% and 57% of the variability in catches of juvenile, small, medium and large Merluccius capensis, respectively, and 71%, 68%, 81% and 70% of juvenile, small, medium and large Merluccius paradoxus, respectively. The significant effects of temperature, oxygen, depth and geographical position on survey catches of hake of different size groups indicate that survey size structure may be affected by the behavior of both species towards environmental conditions. Greater care should therefore be taken when interpreting hake survey biomass estimates, based on swept area method, especially those that were collected during exceptional unfavourable environmental conditions. It would also be
highly desirable if the oceanographic conditions are collected on each trawl station in order to improve understanding of the linkage between resources and environmental conditions
Original languageEnglish
JournalFisheries Oceanography
Volume26
Issue number6
Pages (from-to)680-692
ISSN1365-2419
DOIs
Publication statusPublished - 2017

Cite this

@article{43bdfc7917244e1bb0ae72616dd871a4,
title = "Effects of environmental variables on survey catch rates and distribution by size of shallow- and deep-water Cape hakes, Merluccius capensis and Merluccius paradoxus off Namibia",
abstract = "In order to study the effects of temperature, oxygen, salinity and time of day onsurvey trawl catches, we modeled observed catches of juvenile, small, medium and large hakes per station as functions of zenith angle of the sun, geographical position, year, temperature, salinity, oxygen and depth. We used data from summer demersal surveys conducted during the period 2002–2015, together with a computation of the corresponding light level data from which the solar zenith angles were obtained, and fitted the generalized additive models to these data. Based on best model results, important covariates were oxygen, depth, geographical position and temperature. The best models explained 70{\%}, 69{\%}, 57{\%} and 57{\%} of the variability in catches of juvenile, small, medium and large Merluccius capensis, respectively, and 71{\%}, 68{\%}, 81{\%} and 70{\%} of juvenile, small, medium and large Merluccius paradoxus, respectively. The significant effects of temperature, oxygen, depth and geographical position on survey catches of hake of different size groups indicate that survey size structure may be affected by the behavior of both species towards environmental conditions. Greater care should therefore be taken when interpreting hake survey biomass estimates, based on swept area method, especially those that were collected during exceptional unfavourable environmental conditions. It would also behighly desirable if the oceanographic conditions are collected on each trawl station in order to improve understanding of the linkage between resources and environmental conditions",
author = "Kainge, {Paulus Inekela} and {van der Plas}, {A. K.} and Bartholomae, {C. H.} and Kai Wieland",
year = "2017",
doi = "10.1111/fog.12227",
language = "English",
volume = "26",
pages = "680--692",
journal = "Fisheries Oceanography Online",
issn = "1365-2419",
publisher = "Wiley-Blackwell",
number = "6",

}

Effects of environmental variables on survey catch rates and distribution by size of shallow- and deep-water Cape hakes, Merluccius capensis and Merluccius paradoxus off Namibia. / Kainge, Paulus Inekela; van der Plas, A. K.; Bartholomae, C. H.; Wieland, Kai.

In: Fisheries Oceanography, Vol. 26, No. 6, 2017, p. 680-692.

Research output: Contribution to journalJournal articleResearchpeer-review

TY - JOUR

T1 - Effects of environmental variables on survey catch rates and distribution by size of shallow- and deep-water Cape hakes, Merluccius capensis and Merluccius paradoxus off Namibia

AU - Kainge, Paulus Inekela

AU - van der Plas, A. K.

AU - Bartholomae, C. H.

AU - Wieland, Kai

PY - 2017

Y1 - 2017

N2 - In order to study the effects of temperature, oxygen, salinity and time of day onsurvey trawl catches, we modeled observed catches of juvenile, small, medium and large hakes per station as functions of zenith angle of the sun, geographical position, year, temperature, salinity, oxygen and depth. We used data from summer demersal surveys conducted during the period 2002–2015, together with a computation of the corresponding light level data from which the solar zenith angles were obtained, and fitted the generalized additive models to these data. Based on best model results, important covariates were oxygen, depth, geographical position and temperature. The best models explained 70%, 69%, 57% and 57% of the variability in catches of juvenile, small, medium and large Merluccius capensis, respectively, and 71%, 68%, 81% and 70% of juvenile, small, medium and large Merluccius paradoxus, respectively. The significant effects of temperature, oxygen, depth and geographical position on survey catches of hake of different size groups indicate that survey size structure may be affected by the behavior of both species towards environmental conditions. Greater care should therefore be taken when interpreting hake survey biomass estimates, based on swept area method, especially those that were collected during exceptional unfavourable environmental conditions. It would also behighly desirable if the oceanographic conditions are collected on each trawl station in order to improve understanding of the linkage between resources and environmental conditions

AB - In order to study the effects of temperature, oxygen, salinity and time of day onsurvey trawl catches, we modeled observed catches of juvenile, small, medium and large hakes per station as functions of zenith angle of the sun, geographical position, year, temperature, salinity, oxygen and depth. We used data from summer demersal surveys conducted during the period 2002–2015, together with a computation of the corresponding light level data from which the solar zenith angles were obtained, and fitted the generalized additive models to these data. Based on best model results, important covariates were oxygen, depth, geographical position and temperature. The best models explained 70%, 69%, 57% and 57% of the variability in catches of juvenile, small, medium and large Merluccius capensis, respectively, and 71%, 68%, 81% and 70% of juvenile, small, medium and large Merluccius paradoxus, respectively. The significant effects of temperature, oxygen, depth and geographical position on survey catches of hake of different size groups indicate that survey size structure may be affected by the behavior of both species towards environmental conditions. Greater care should therefore be taken when interpreting hake survey biomass estimates, based on swept area method, especially those that were collected during exceptional unfavourable environmental conditions. It would also behighly desirable if the oceanographic conditions are collected on each trawl station in order to improve understanding of the linkage between resources and environmental conditions

U2 - 10.1111/fog.12227

DO - 10.1111/fog.12227

M3 - Journal article

VL - 26

SP - 680

EP - 692

JO - Fisheries Oceanography Online

JF - Fisheries Oceanography Online

SN - 1365-2419

IS - 6

ER -