Life history, habitat utilisation, and biomass of introduced Mysis relicta

AbstractLife history, habitat utilisation, and biomass of benthic and pelagic opossum shrimp (Mysis relicta) were studied in the oligotrophic Lake Jonsvatn, central Norway. Sampling in the pelagic zone was done by means of a closing zooplankton net; in the benthic zone by means of a benthic beam trawl.M. relicta had a mixed one or two year life cycle. In the autumn, the proportion of mature females and males were larger in the pelagic than in the benthic habitat. Copulation took place in late autumn, and the first females with eggs occurred in November. In February, the first juvenile M. relicta were released in the benthic habitat. In May and July, however, juveniles were found in large numbers in all parts of the lake. The length distribution of M. relicta indicates that juveniles partly segregate between benthic and pelagic habitats.Both juvenile and adult M. relicta performed vertical diel migrations in the pelagic habitat. In the benthic habitat, diel vertical migrations along the bottom were not as pronounced as vertical migrations in the pelagic habitat. In the benthic habitat, major migrations were performed only by adults in the autumn. Our results indicate that the light intensity in the green part of the spectrum gives the proximate cue for regulation of vertical distribution of M. relicta.The mean total biomass varied between 288 and 1576 kg dry weight, corresponding to 23.2–127.1 mg dry weight m−2 surface area. M. relicta had smallest biomass during late spring/early summer and largest biomass during autumn and early winter. Estimated pelagic biomasses were largest in February, August, October and November, while benthic biomasses were largest in May and July. Estimated biomass of pelagic M. relicta during autumn was approximately 1/10 of the estimated biomass of zooplankton in this lake

Fish populations, gill net selectivity, and artisanal fisheries in the Okavango River, Namibia. Recommendations for a sustainable fishery.

Hay, C.J., Næsje, T.F., Breistein, J., Hårsaker, K., Kolding, J., Sandlund, O.T. & van Zyl, B. 2000. Fish populations, gill net selectivity, and artisanal fisheries in the Okavango River, Namibia. Recommendations for a sustainable fishery. — NINA-NIKU Project Report 010: 1-105. Objective: The objective of this report is to produce guidelines for a sustainable management of the fisheries in the Okavango River, Namibia, based on fish survey data for the years 1992-1999. Data were also coHected regarding the subsistence fishery in the river. The rationale for the report is the 1995 White Paper "Responsible Management of the Inland Fisheries of Namibia" and the draft bill on inland fisheries. These have the objectives of ensuring a sustainable and optimal utilisation of the freshwater resources, and to favour utilisation of fish resources by subsistence households over commercialisation. The stated policy takes into consideration the large differences among water systems in Namibia and proposes adoption of sepa rate management regimes for the various river systems. Material and study area: Fish were collected in seven main areas (Matava, Musese, Bunya, Rundu, Cuito, Mbambi, and Kwetze) with survey gill nets (22-150 mm stretched mesh) and 16 other sampling methods, such as seine nets, mosquito nets, cast nets, angling, electrofishing apparatus, rotenone and different traditional gears. These are later collectively called "other gears". The stations were selected to include all main habitats present in the Okavango River. The gill nets were mainly used in relatively deep water, whereas the other gears were mainly used in shallow and vegetated habitats. All seven localities were sampled at least once a year between 1992 and 1999. A total of 47438 fishes were sampled, 13559 in gill nets, and 33879 in other gears. The Okavango River originates in the central highlands of Angola at approx. 1700 m a.s.l. and enters Namibia at Katwitwi. The river forms the border between Namibia and Angola before turning south towards Botswana. Along the Namibian section of the river, there are large floodplains with sandy substrates and rocky outcrops, and abundant aquatic vegetation. After a distance of 460 km within Namibia, the river enters Botswana where it evaporates in the swamps of the Okavango Delta. The annual flood in the Namibian portion of the Okavango starts during December reaches its peak in March-April and recedes during May. The annual discharge of the Ol avango at Rundu is between 5,000 and 6,000 million m3. Below the confluence with the Cuito River, the annual discharge nearly doubles to over 10,000 million m3. More than 136,000 people live in the Okavango region. An estimated 90 % of the population live within 10 km of the river, and many of the economic and social activities in the region are connected to the river. More than 50 % of the human population along the river do fish, both with traditional gears such as baskets, funnel traps and fences constructed of plant material, and with some modern gears such as gill nets, seines, hook and line, and mosquito-nets. For more than 90 % of the households fish is a source of subsistence, and sales of fish provide some income for approximately 45 % of the households. The highest population density is at Rundu, whereas no people live at Kwetze, which is within the Mahongo Game Park. It is assumed that the fishing pressure is correlated to the riparian population density

Fish populations, gill net selectivity, and artisanal fisheries in the Okavango River, Namibia. Recommendations for a sustainable fishery.

Hay, C.J., Næsje, T.F., Breistein, J., Hårsaker, K., Kolding, J., Sandlund, O.T. & van Zyl, B. 2000. Fish populations, gill net selectivity, and artisanal fisheries in the Okavango River, Namibia. Recommendations for a sustainable fishery. — NINA-NIKU Project Report 010: 1-105. Objective: The objective of this report is to produce guidelines for a sustainable management of the fisheries in the Okavango River, Namibia, based on fish survey data for the years 1992-1999. Data were also coHected regarding the subsistence fishery in the river. The rationale for the report is the 1995 White Paper "Responsible Management of the Inland Fisheries of Namibia" and the draft bill on inland fisheries. These have the objectives of ensuring a sustainable and optimal utilisation of the freshwater resources, and to favour utilisation of fish resources by subsistence households over commercialisation. The stated policy takes into consideration the large differences among water systems in Namibia and proposes adoption of sepa rate management regimes for the various river systems. Material and study area: Fish were collected in seven main areas (Matava, Musese, Bunya, Rundu, Cuito, Mbambi, and Kwetze) with survey gill nets (22-150 mm stretched mesh) and 16 other sampling methods, such as seine nets, mosquito nets, cast nets, angling, electrofishing apparatus, rotenone and different traditional gears. These are later collectively called "other gears". The stations were selected to include all main habitats present in the Okavango River. The gill nets were mainly used in relatively deep water, whereas the other gears were mainly used in shallow and vegetated habitats. All seven localities were sampled at least once a year between 1992 and 1999. A total of 47438 fishes were sampled, 13559 in gill nets, and 33879 in other gears. The Okavango River originates in the central highlands of Angola at approx. 1700 m a.s.l. and enters Namibia at Katwitwi. The river forms the border between Namibia and Angola before turning south towards Botswana. Along the Namibian section of the river, there are large floodplains with sandy substrates and rocky outcrops, and abundant aquatic vegetation. After a distance of 460 km within Namibia, the river enters Botswana where it evaporates in the swamps of the Okavango Delta. The annual flood in the Namibian portion of the Okavango starts during December reaches its peak in March-April and recedes during May. The annual discharge of the Ol avango at Rundu is between 5,000 and 6,000 million m3. Below the confluence with the Cuito River, the annual discharge nearly doubles to over 10,000 million m3. More than 136,000 people live in the Okavango region. An estimated 90 % of the population live within 10 km of the river, and many of the economic and social activities in the region are connected to the river. More than 50 % of the human population along the river do fish, both with traditional gears such as baskets, funnel traps and fences constructed of plant material, and with some modern gears such as gill nets, seines, hook and line, and mosquito-nets. For more than 90 % of the households fish is a source of subsistence, and sales of fish provide some income for approximately 45 % of the households. The highest population density is at Rundu, whereas no people live at Kwetze, which is within the Mahongo Game Park. It is assumed that the fishing pressure is correlated to the riparian population density