M Oliver

Mesozooplankton biomass, composition, and potential grazing pressure on phytoplankton during austral winter and spring 1993 in the subtropical convergence region near New Zealand

The biomass, composition, and grazing rates of three size fractions of mesozooplankton (200–500, 500–1000, and some >1000 μm) were estimated in shelf waters and the water masses associated with Subtropical Convergence east of New Zealand, in the austral winter and spring of 1993, as part of a larger New Zealand study of ocean carbon flux that contributes to the Joint Global Ocean Flux Study (JGOFS). The total biomass was largest in spring in all water types. It was similar to the biomass measurements made previously in subantarctic and subtropical water masses in the Southwest Pacific and those from the North Atlantic, except for the spring biomass in subtropical water which was unusually large (86.5 and 101.3 mg m-3 dry weight). Biomass was concentrated in the upper 100 m, especially within the 0–25 or 25–50 m layers, both day and night. Night/day biomass ratios in the surface 100 m were often >2, and are presumed to be the result of sampling patchy populations as well as vertical migration. Biomass was greatest for the >1000 μm fraction of the mesozooplankton population, followed by the 500–1000, and 200–500 μm fractions, respectively. The unusually small fraction of biomass residing in the 200–500 μm fraction is assumed to be the result of predation by larger mesozooplankton. The mesozooplankton community had maximum gut fluorescence at night only at stations where chlorophyll a was >2 mg m-3 and at many of the stations gut fluorescence was persistently low. This was probably the result of the poor feeding environment, since a large proportion of the primary production resided in the <2 μm fraction. The total meaningestion of phytoplankton was calculated to be 1–40 mgC m-2 d-1, based mainly on ingestion by the 200–500 and 500–1000 μm fractions, which were dominated by herbivores or herbivores and omnivores. The heaviest grazing pressure was in subtropical and Subtropical Convergence waters, in spring. Total grazing represented <1–4% of daily total integrated primary production. Phytoplankton carbon ingested usually met only a small fraction of the basic metabolic requirements of the mesozooplankton. These data, and the fact that spring populations were apparently actively growing, since they contained a large proportion of developmental stages, imply that mesozooplankton diets were mainly microzooplankton.

Stock enhancement of rock lobsters (Jasus edwardsii) : timing of predation on naïve juvenile lobsters immediately after release

Abstract The success of enhancement programmes hinges on the survival of released animals. One factor greatly influencing short‐term survival of reseeded lobsters is the timing and intensity of predation relative to the time of release. The activity and abundance of predators varies over daily, seasonal, and annual scales and knowing the best time to release juveniles will minimise mortality. We used chronographic tethering devices and remote video equipment at 10 sites near Wellington, New Zealand and Hobart, Tasmania, Australia, to assess the relative timing and intensity of predation for released lobsters. Our studies showed that predation was greatest within the first 2 h after release (χ 2 = 60.425, d.f. = 9, P < 0.001) suggesting that the disturbance associated with the release itself may draw the unwanted attention of predators. Relative predation rates also peaked on each of the following two mornings, possibly because of the emergence of daytime predators. The video footage obtained at the Tasmanian sites revealed that most predation was by fish (46%), but surprisingly, cannibalism comprised 16% of predation events. The limitations of tethering as a method are discussed in numerous reviews but proved useful as a relative measure for these highly mobile and cryptic animals. Further consideration needs to be given to methods of release that minimise mortality of recently seeded lobsters.

Validation of SeaWiFS data from around New Zealand

Abstract The near-surface concentration of chlorophyll- a ( C a ) was measured during 11 research voyages around New Zealand between 1998 and 2000. Over 400 measurements of spectral normalised water-leaving radiance were made using a high precision profiling spectroradiometer. The data were used to assess the quality of remotely sensed measurements of C a by the NASA ocean colour sensor, SeaWiFS, in New Zealand waters. The results in the open-ocean regions are consistent with a combination of negative biases in the C a algorithm of ca. 20% (some regional dependency), and positive biases due to under-estimation of water reflectance towards the blue end of the spectrum by SeaWiFS.

Anti-Predator Behavior of Captive-Reared and Wild Juvenile Spiny Lobster (Jasus edwardsii)

Early juvenile lobsters reared in captivity may loose anti-predator behaviors displayed by their wild counterparts. To test this hypothesis in juvenile spiny lobsters (Jasus edwardsii), we conducted a series of experiments in which recently settled pueruli were reared in captivity under differing levels of predation risk. After one year in captivity, these lobsters were exposed to a predator in a tank or released into the wild to assess their ability to recognize and respond to predation. We found that lobsters raised without predators significantly reduced their movement activity outside shelters when subsequently introduced to a predator compared with juveniles raised with predators (F1,45 = 4.33, p < 0.05). This apparent over-compensation may be necessary for lobsters to learn the appropriate anti-predator response. Lobsters raised without predators and released into the wild displayed the same types of behaviors as resident wild lobsters. Released lobsters spent more time defending and fighting amongst themselves than the wild lobsters (G = 13.006 > χ2 0.05,2 = 9.488), but they displayed the appropriate anti-predator responses when approached or attacked by a predator. Our results show that juvenile spiny lobsters habituate to the absence of a predator when reared in captivity for long periods, but display an innate ability to recognize and effectively respond to predators when encountered in a hatchery environment or in the wild.

Spiny lobster population enhancement: moderation of emergence behaviour of juvenile Jasus edwardsii reared in captivity

Abstract Lobster stock enhancement efforts have generally met with limited success owing, in part, to low survival of reseeded juveniles caused by poor development of anti‐predator behaviour. We found that recently‐settled juvenile lobsters reared in captivity for 1 year exhibit significantly higher levels of daytime activity than their like‐sized wild counterparts and that this daytime activity could be reduced by 50% by either rearing the lobsters with a predator or by feeding them at night. In combination, predator presence and night‐time feeding further reduced daytime emergence to low levels. However, a field experiment in which we observed the behaviour and emergence times of captive‐reared lobsters immediately after release on reefs where predators were abundant, showed that regardless of rearing conditions and associated changes in behaviour, lobsters assess the risk of predation and quickly assume nocturnal emergence and activity patterns. This plasticity in behaviour may enable juveniles to maximise foraging range and duration in response to predation pressure.

Vertical distribution of zooplankton > 39 μm in relation to the physical environment off the west coast of South Island, New Zealand

Abstract Data sets of pump‐sampled zooplankton > 39 μm were obtained for early August 1987, late July 1988, and late June 1990, along with environmental data in 1987 and 1990, off the west coast of South Island, New Zealand. The environmental circumstances favourable to zooplankton species known to be food for hoki larvae (Macruronus novaezelandiae) were investigated. We found that a major proportion of the vertical distribution of the epipelagic zooplankton off Westland is influenced by passive interaction with physical processes against a background of the distribution of autotrophic particles. Multivariate analysis classified stations into continental slope (Group I), outer‐mid shelf (Group II), and neritic (Group III) stations/depths. Contrasting with other stations in Group I (the habitat of hoki larvae), the physical conditions of one station differed in that winter mixing had hardly begun (indicated by relatively low nutrients), concentrations of 20–200 μrn autotrophic particles were low, as were c...

Conservation of unique patterns of body markings at ecdysis enables identification of individual spiny lobster, Jasus edwardsii

Abstract Southern temperate rock lobsters (Jasus edwardsii) develop complex body markings and spines that vary uniquely among individuals. These markings are retained after moulting allowing individuals to be recognised without capture or handling in the field. This enables the impact of capture, handling, and tagging procedures on subsequent movements to be more confidently evaluated. Recognition of individuals by natural markings allows post‐moult lobsters that were externally tagged and held in large communal tanks to be correctly retagged. Distinctive markings on the pedate processes and on the epistoma, as well as the number and placement of small spines on the antennular plate are particularly useful for recognising individuals. It is highly likely that the other Jasus species and the brightly patterned and coloured Panulirus species have similar body markings and spine patterns that would aid recognition of individuals.