Rodney D. Roberts

Factors affecting the food value of diatom strains for post-larval abalone Haliotis iris

The growth and survival of post-larval abalone Haliotis iris (mean initial shell length 570 μm) fed on eight strains (seven species) of benthic diatom were examined in the laboratory. Post-larvae showed active feeding behaviour on all diatom strains. Two strains (Achnanthes longipes-1 and Nitzschia sp.) produced significantly faster growth (means 34–35 μm shell length per day) than the remaining six strains (means 11–17 μm/day). The two fast-growth strains were efficiently digested by post-larvae, with 93–94% of live cells ruptured during passage through the gut. Two of the remaining strains could not be ingested during the growth experiment because the cells were too large (A. longipes-2) or too strongly attached (Cocconeis pseudomarginata). For the other four strains (Navicula britannica, Nav. ramosissima, Navicula sp. and Nitzschia ovalis) most cells passed through the gut alive. Survival rate was highest on the two fast-growth strains, and survival was positively correlated with both growth rate (r=0.73, P<0.05) and digestion efficiency (r=0.80, P<0.05) of diatoms. The digestion efficiency of diatom strains appears to be an important factor determining their dietary value. Digestion efficiency can be influenced by diatom morphology, attachment strength, frustule strength and post-larval age/size. Isolation and use of digestible strains may improve hatchery culture of abalone.

PRIMARY PRODUCTION OF CRUSTOSE CORALLINE RED ALGAE IN A HIGH ARCTIC FJORD1

Crustose coralline algae occupied ∼1%–2% (occasionally up to 7%) of the sea floor within their depth range of 15–50 m, and they were the dominant encrusting organisms and macroalgae beyond 20 m depth in Young Sound, NE Greenland. In the laboratory, oxygen microelectrodes were used to measure net photosynthesis (P) versus downwelling irradiance (Ed) and season for the two dominant corallines [Phymatolithon foecundum (Kjellman) Düwel et Wegeberg 1996 and Phymatolithon tenue (Rosenvinge) Düwel et Wegeberg 1996] representing> 90% of coralline cover. Differences in P‐Ed curves between the two species, the ice‐covered and open‐water seasons, or between specimens from 17 and 36 m depth were insignificant. The corallines were low light adapted, with compensation irradiances (Ec) averaging 0.7–1.8 μmol photons·m−2·s−1 and light adaptation (Ek) indices averaging 7–17 μmol photons·m−2·s−1. Slight photoinhibition was evident in most plants at irradiances up to 160 μmol photons·m−2·s−1. Photosynthetic capacity (Pm) was low, averaging 43–67 mmol O2·m−2 thallus·d−1 (∼250–400 g C·m−2 thallus·yr−1). Dark respiration rates averaged ∼5 mmol O2·m−2 thallus·d−1. In ice covered periods, Ed at 20 m depth averaged ∼1 μmol photons·m−2·s−1, with daily maxima of 2–3 μmol photons·m−2·s−1. During the open water season, Ed at 20 m depth averaged ∼7 μmol photons·m−2·s−1 with daily maxima of ∼30 μmol photons·m−2·s−1. Significant net primary production of corallines was apparently limited to the 2–3 months with open water, and the small contribution of corallines to primary production seems due to low Pm values, low in situ irradiance, and their relatively low abundance in Young Sound.

A method for inducing muscle relaxation in the abalone, Haliotis iris.

Abstract Four chemical and two non-chemical methods for induction of relaxation in the abalone, Haliotis iris, were tested for prospective use in pearl seeding. Requirements were for rapid onset of relaxation, full extension and softening of the foot and epipodium muscle for at least 10 min, and recovery with minimal stress and no mortality. Nembutal at 1 ml/l was most successful, producing relaxed and soft muscle with good accessibility to operation sites, and no mortality. Benzocaine and MS-222 made animals release from container walls within 30–60 min, but the animals muscle was hard and contracted. Propylene phenoxytol caused muscle contraction and mortality at 2.5 ml/l. The effects of the chemical relaxants were similar in warm (21–22.5°C) and cool (15–16°C) water. Elevation of the water temperature from 18°C to 23°C (without chemicals) caused some softening of the muscle, but was much less effective than Nembutal. Air exposure for up to 4 h was ineffective. This work provides a simple method to relax live abalone for pearl nucleation or other manipulations.

Towards cryopreservation of Greenshell mussel (Perna canaliculus) oocytes.

Cryopreservation is a powerful tool for selective breeding in aquaculture as it enables genetic material from selected stock to be stored and crossed at will. The aim of this study was to develop a method for cryopreserving oocytes of the Greenshelltrade mark mussel (Perna canaliculus), New Zealands main aquaculture species. The ability of oocytes to be fertilized post-thawing was used as the criterion for success in initial experiments and then subsequently, the ability of frozen oocytes to develop further to D-stage larvae was assessed. Ethylene glycol, propylene glycol, dimethyl sulphoxide and glycerol were evaluated at a range of concentrations with and without the addition of 0.2M trehalose using post-thaw fertilization as the endpoint. Ethylene glycol was most effective, particularly when used in combination with trehalose. A more detailed investigation revealed that ethylene glycol at 9% or 10% in the presence of 0.2-0.4M trehalose afforded the best protection. In experiments varying sperm to egg ratio and egg density in post-thaw fertilization procedures, D-larval yield averaged less than 1%. Following these results, a detailed experiment was conducted to determine the damaging steps in the cryopreservation process. Fertilization losses occurred at each step whereas D-larval yield approximately halved following CPA addition and was almost zero following cooling to -10 degrees C. Cryomicroscopy studies and fertilization results suggest that the inability of oocytes to develop to D-larvae stage after cooling to -10 degrees C and beyond are most likely related to some form of chilling injury rather than extracellular ice triggering intracellular ice formation. Further research is needed to determine the causes of this injury and to reduce CPA toxicity and/or osmotic effects.

FACTORS AFFECTING SETTLEMENT OF ABALONE (HALIOTIS IRIS) LARVAE ON BENTHIC DIATOM FILMS

Abstract Diatoms are widely used in abalone hatcheries to induce larval settlement (=attachment and metamorphosis) but there are few data on the factors that influence their effectiveness as settlement cues. Of 16 diatom strains tested in Experiment 1, half induced ≥80% attachment and >50% metamorphosis within 4 days. Settlement success did not correlate significantly with diatom abundance or adhesive strength (P > 0.05). Several diatom strains interfered with settlement. Examples included (1) smothering by highly mobile diatoms (Nitzschia longissima); (2) shells becoming stuck to sticky secretions (Navicula britannica, Achnanthes longipes); and (3) unstable diatom cells preventing pedal attachment (Licmophora sp.). In Experiment 2, only 1 of 15 diatom strains induced over 70% metamorphosis, and three others induced 22–36% metamorphosis, within 4 days. Older cultures of a strain induced higher attachment and metamorphosis than younger cultures of the same strain (P < 0.001), but some old cultures still gave poor settlement. For the young diatom cultures, larval attachment correlated positively with diatom percent cover (r = 0.89, P < 0.05), and metamorphosis with the growth phase of the diatom film (r = 0.91, P < 0.05). Experiments 3–5 examined the role of bacteria in settlement induction by diatom films. In Experiment 3, films of 8 diatom species from Experiment 1 were regrown from ∼6 individually isolated and rinsed cells, likely altering the associated bacterial flora. Metamorphosis was 179 fold lower on average than in Experiment 1 (P < 0.0001), but attachment and diatom density were not significantly different (P = 0.87 and P = 0.75 respectively). In Experiment 4, Nitzschia ovalis grown and assayed with antibiotics had lower metamorphosis after 2 days than the same strain grown and assayed without antibiotics (6 vs 64%, P = 0.004), whereas attachment after 2 days did not differ (94 versus 92%, P = 0.49). In Experiment 5 bacteria from a Nitzschia ovalis culture induced as much attachment and metamorphosis as the intact diatom film, whereas cell-free supernatant was much less effective. The presence of antibiotics in settlement assays reduced attachment and metamorphosis by biofilms, but not by coralline algae (Phymatolithon repandum) or GABA. In Experiment 5 metamorphosis on diatoms and bacteria occurred gradually over 2 wk, whereas on coralline algae and GABA it occurred within a few days. This study suggests that many diatoms cue rapid larval attachment, but few induce consistently strong metamorphosis within 4 days in laboratory conditions. The bacteria present in diatom films affect the settlement-inducing activity of the film in at least some cases. The effectiveness of diatom films was generally higher for more mature films but overall, physical factors explained little of the variation in the activity of diatoms.

VIABILITY OF ABALONE (HALIOTIS IRIS) STOCK ENHANCEMENT BY RELEASE OF HATCHERY-REARED SEED IN MARLBOROUGH, NEW ZEALAND

Abstract The total allowable commercial catch from New Zealands Haliotis iris Gmelin 1791 fishery was reduced by 18% between 1999 and 2004. Quota holders have initiated research to assess the viability of stock enhancement by release of hatchery-reared seed. Boulder reefs (1 × 2 × 0.5 m) were constructed by placing natural boulders in wire baskets over sand or bedrock. These reefs allowed accurate census of small abalone during short-term experiments (3–4.5 mo.) examining the effects of seed size [5–25 mm shell length (SL)] and density (25–640 m−2 of seafloor) on survival and growth. Survival increased with seed size, but beyond 10 mm the increased survival did not offset the cost of larger seed. Growth and survival of 8–24 mm seed decreased with increasing density, but regressions were nonsignificant (P = 0.06–0.36) because of variability among reefs. Densities as high as 300 m−2 gave good growth and survival (>40%) over 3 mo. on some reefs. Five natural sites were seeded with 2,600–20,000 juveniles (mean 10–11 mm SL, range 6–19 mm) at an average density of 50 m−2 to estimate long-term survival from commercial reseeding. After 17–20 mo, when recovered abalone averaged 47–60 mm SL, survival varied widely among the five sites ranging from 1.7% to 25.1% (average 13.8%). Estimated survival to harvest size of 125 mm SL ranged from 1.3% to 18.6% (average 10.2%) assuming 3 further years of mortality at M = 0.1. The two sites with the lowest survival were affected by substrate movement during storms, highlighting the risk of using exposed locations with boulders small enough to be turned for surveys. Survival to harvest averaged 15.2% across the three sites without significant storm damage. Growth averaged 29.5 mm SL year−1 across the five sites (range 25–33 mm·year−1). A model was used to examine the economic viability of reseeding, assuming that reseeded abalone supplement natural recruits. At a price of NZ

Minimal impact from long‐term dredge spoil disposal at a dispersive site in Tasman Bay, New Zealand

0.32 per 10 mm SL seed, the return on investment was 20% yr−1 at 10% survival to harvest, and 30% yr−1 at 15% survival. These returns compare favorably with opportunity costs of ∼10% yr−1, suggesting that reseeding is likely to be economically viable if sites and habitat are carefully selected. Large scale seeding should be accompanied by monitoring to quantify net population increase.

EFFECT OF WATER VELOCITY AND BENTHIC DIATOM MORPHOLOGY ON THE WATER CHEMISTRY EXPERIENCED BY POSTLARVAL ABALONE

Abstract The effects of dredge spoil disposal on contaminant concentrations and benthic macrofauna were examined at a shallow marine disposal site used for c. 20 years. The site had received c. 50 000 m3 yr‐1 of maintenance dredgings annually, from the Port of Nelson. Port sediments were contaminated to varying degrees with some trace metals, organo‐chlorine pesticides, polychlorinated biphenyls, and polycyclic aromatic hydrocarbons. They showed mildly elevated toxicity in laboratory bioassays, and their macrofauna was dominated by small‐bodied polychaetes. Despite this, there was very little indication of impact in the spoil disposal area. The disposal area, spoil spreading zone and control sites were all similar in terms of sediment contaminants, sediment toxicity, neogastropod imposex, and macrofauna. The lack of discernible impact is probably because of the dynamic sedimentary environment in the disposal area, which disperses dumped dredgings and mixes them with ambient sediment.

SETTLEMENT OF HALIOTIS AUSTRALIS LARVAE: ROLE OF CUES AND ORIENTATION OF THE SUBSTRATUM

Abstract The water bathing postlarval abalone often lies within the diffusive boundary layer (DBL) so its chemistry is greatly influenced by the metabolism of the biofilm on which the abalone feed. This study used microelectrodes to investigate the influence of water velocity and diatom morphology on dissolved oxygen and pH in the DBL. Decreasing water velocity increased the thickness of the DBL, thereby increasing the amplitude of changes in oxygen concentration. Over a film of the prostrate diatom Nitzschia ovalis Arnot, DBL thickness averaged 71, 139, 177, and 406 μm at water velocities of 78, 15, 7, and 1 mm s−1 respectively. Corresponding oxygen concentrations at the biofilm surface under moderate light (75 μE m−2 s−1) and temperature (15°C) averaged 111%, 120%, 125%, and 151% of air saturation respectively, at the four velocities. The presence of a 1-mm tall diatom canopy (Achnanthes longipes Agardh) over a Nitzschia ovalis film thickened the DBL by 3-fold at 1 mm s−1 and 6-fold at ∼80 mm s−1. The thickened DBL and higher diatom biomass generated extreme conditions at the biofilm surface. Dissolved oxygen concentrations as high as 440% of air saturation, and pH as high as 9.8 were recorded beneath the canopy in moderate light (105 μE m−2 s−1) and temperature (15°C) at a water velocity of 1 mm s−1. Changes during darkness were less extreme, with 53% oxygen saturation and pH 7.7 the minima recorded. These measurements demonstrate the extreme water chemistry that can develop in the microhabitat of postlarval abalone. The changes will be amplified by the presence of filamentous diatoms, by increased light intensity, and by lack of water movement. Standard aeration will greatly reduce the extremes experienced by postlarvae by generating water movement sufficient to thin the DBL.

Is Settlement of Haliotis iris Larvae on Coralline Algae Triggered by the Alga or Its Surface Biofilm

ABSTRACT Haliotis australis is a medium-size New Zealand abalone with aquaculture potential. H. australis can be readily ripened and spawned, but larval settlement has proved difficult. The current study addressed aspects of larval settlement (i.e., attachment and metamorphosis) in laboratory experiments and small-scale hatchery trials. Competence to metamorphose was first seen weakly in larvae at 8 days old at 14.5°C, but larval metamorphosis rates increased progressively up to at least 12 days of age. In laboratory experiments, larvae showed a dose-dependent response to dissolved &ggr;-aminobutyric acid (GABA), with settlement peaking at 1 µM (57% metamorphosis 4 days after settlement induction) and declining at higher and lower doses. Across a range of cues, metamorphosis was highest on crustose coralline algae (97 ± 4% metamorphosis after 4 days), intermediate on 1 µM GABA (55 ± 14%), and low on diatom films (<20%). GABA combined additively with diatom film to give strong metamorphosis (81 ± 6%). Metamorphosis induction by a diatom film was doubled if the film was pregrazed by juvenile H. australis (76% vs. 35%, P < 0.0001), but the grazing effect appeared to be species specific, because pregrazing by Haliotis iris had no effect (41% vs. 35%, P = 0.720). The species-specific response to trail mucus appears to preclude GABA as being the main inducer of metamorphosis in this case. On hatchery plates, settlement on pregrazed biofilms was approximately 4-fold higher when the plates were horizontal rather than vertical (41 % vs. 10% metamorphosis, P = 0.003). Settlement on ungrazed horizontal plates remained low even if those plates had been held in water with adult conspecifics, but without contacting the abalone. We conclude that tanks offering sloped or horizontal surfaces will be more effective than vertical plates for settlement of H. australis, and that pregrazed biofilms and/or GABA are promising settlement cues.