A Proposal for the Development of Halibut Culture in the Lower Bay of Fundy, New Brunswick

Canadian Halibut Inc.

Table of contents

• Introduction
• Objective
• Methods
• Study Design
• Fish Tagging
• Water Quality Monitoring
• Water Temperature Monitoring
• Dissolved Oxygen Monitoring
• Follow-up Monitoring
• Results
• Post Transfer Mortality
• Water Quality, Temperature & Dissolved Oxygen
• Conclusions
• Feasibility of Use
• Economics
• Summary of Conclusions
• References

Introduction

Transport is considered the most stressful and necessary component in the production cycle of most aquaculture species. This short period generally consisting of 12-72 hours is by far the most risky and sensitive period.  The cumulative effect of transport related stress can reveal short cuts taken in the days, years and months prior to transport, alternatively the cumulative effect of stress can lead to poor production and elevated mortality post transfer. In the past fish transport has developed almost exclusively with experience of the industry, with limited scientific inputs from basic and pure sciences.

As the number of novel aquaculture species continues to expand so does the distance between hatcheries, nurseries and grow out farms continues. This combination of distance and new species biology highlights the need for novel transport methods. For this reason more and more research programs on transport have developed in recent years.

Currently the equipment available for the transport of live fish in Atlantic Canada has been optimized for the transport of Atlantic salmon (Salmon salar). This equipment is readily available in the region it is a natural choice for for the necessary transport of Atlantic halibut juveniles. Currently there is a single producer of halibut juveniles that is geographically separated from commercial grow out sites, the only feasible transport method of juveniles of appropriate size for cage site stocking is overland (opposed to boat or air).

Atlantic halibut are a negatively buoyant flatfish with considerably different transport requirements than neutrally buoyant Atlantic salmon.  High densities within transport tanks are required to make transport economically feasible at industry scales, the buoyancy issue makes halibut transport a challenge.  Atlantic salmon have the ability to distribute themselves through the water column within transfer tanks, where as halibut require surface area to settle. By nature halibut have relatively low oxygen demands and low metabolic rates lending themselves as good candidates for transport given that are provided surface area to settle. In the absence of suitable area they have one of two options. They can settle on top of one another increasing the percent covered area (PCA) within tanks.  If settlement reaches critical depths, localized areas of suboptimal conditions may develop (Brown 2002) (Reig et al. 2007). This localized crowding can lead to suffocation, increased stress and the potential for damage to skin, mucous and eyes as fish rub against one another.  The second option is to remain swimming in order to hold a position within the water column. This Increased activity is contrary to the normal behavior the species and will cause unnecessary stress to the fish in addition to substantially increasing O2 consumption and subsequent CO2 production which can have negative impact on water quality parameters.

The purpose of this study is to investigate the effect of decreasing the percent covered area (PCA) within transfer tanks on the post-transfer survival of Atlantic halibut (Hippoglossus hippoglossus) juveniles. Surface area has a direct and inverse relationship with the PCA (Reig et al. 2007) within tanks.  In simple terms PCA referrers to the depth of fish if all fish were to settle on the available surface area (e.g. 200% coverage = 2 fish deep). This project also attempts to provide insight into the unique transport requirements of halibut with the potential to make the procedure more efficient by enabling an increased stocking density while lowering the PCA within transfer tanks. As Atlantic halibut farming moves towards commercial scales in Atlantic Canada and transportation costs continue to rise, it is important to investigate ideas that will improve fish survival and the overall efficiency of live fish transport technologies for flatfish.

Objective:

To determine if reducing PCA in halibut transfer tanks by providing increased surface are for settlement will reduce post transfer mortality.

Methods

Study Design:

A population of 10,675 550g halibut were transferred from a single land based tank in Advocate, Nova Scotia to a single sea cage in Lime Kiln Bay, New Brunswick. The transfer involved three equal loads using one live haul smolt truck outfitted with five individual 6.25 m³ transfer tanks.  Each tank was outfitted with their own supply of adjustable liquid oxygen and a common air bubbler for mixing of the tank volume. The trial was designed to use the three separate transfers as replicates. During each individual transfer two of the five transfer tanks would be utilized.  One randomly chosen tank would be the control and a second randomly chosen tank would serve as the treatment. Treatment tanks utilized the wire mesh baskets, whereas the controls utilized the current industry method of transportation. Percent area covered within tanks was estimated using the formula describe in Reig et al. 2007.

Fish Tagging:

The treatment and control groups were identified by a temporary mark on the blind side of the fish. Treatment groups received this mark cranially while control groups received the mark caudally. The temporary mark was made by jet injecting alcian blue dye into the skin (Thedinga et al. 1997).

Water Quality  Monitoring:

Water quality samples were collected from below the water surface of the transfer tanks at three time points during the transfer and keep sealed and cool in glass bottles. The first sample was taken after the filling of the tanks and prior to loading the fish. The second sample was taken at the temporal mid-point of the transfer and the final end point was taken just prior to unloading the tanks for both loads two and three. Water was collected and sealed in glass bottles and analyzed by The Department of Fisheries and Oceans, St Andrews Biological Station (Debbie Martin-Robichaud).

Water Temperature Monitoring:

Water temperature within transfer tanks was recorded using Onset Hobo® underwater temperature Loggers. Temperature was recorded on a min-1 basis throughout the transfer.

Dissolved Oxygen Monitoring:

Dissolved oxygen was monitored using a hand held OxyGuard® Handy Polaris dissolved oxygen meter at various every 1.5 -2hr during the transport.

Follow-up Monitoring:

Mortalities were recovered and enumerated by treatment group 34 days following the last transfer as it is typical by industry standards to have most post transfer related mortalities occur during this period. It was also chosen because investigators could be reasonably sure that the temporary mark utilized would be adequately visible during this short follow-up period.

Results

Post Transfer  Mortality:

Mortality dives were conducted on a bi-weekly basis post transfer, as permitted by weather. Peak mortality occurred between 9-17 days post transfer with 6.6% and 10.0% mortality observed in treatment and control groups respectively. Following the 34 day observation period mortality was 11.2%, 14.6% and 37.1% in treatment, control and untagged fish respectively. The treatment and control groups were found to have significantly different Kaplan-Meier survival functions χ2 2(1) = 5.30, p=0.021.

Water Quality, Temperature & Dissolved Oxygen:

Water quality parameters seemed remained within reasonable limits during transport. The pH of tank water was lowest at the mid-point of transport and increased marginally towards neutrality prior to unloading. Salinity remained a stable through the transfer.  No major perturbations in dissolved oxygen concentration were observed in the treatment or control tanks during the two transfers. The temperature of transfer was observe to drop over the course of the transport, with on average a 0.5°C drop in temperature over the 7-8hr transfer from Advocate to Lime Kiln.

Conclusions

A large proportion of the fish were hyper pigmented on the blindside this prevented the ability to create unique marks on the fish by varying the location of the mark on the fish. Due to this restriction only two definitive marker were possible a cranial (head) and a caudal (tail) stamp. Clearly one could suggest to more precisely place the marks (e.g. On blind side operculum) but the hyper pigmentation patterns varied greatly and without the use of an anesthetic to reduce fish movement such precision would be difficult. One could also suggest using additional marks to indicate different treatment/replicate groups. This was considered a good option but wasn’t used because it was considered too costly in terms of time to mark each individual fish with two separate marks.

Measured to control confounding of survival, no major perturbations noted and “normals” for Atlantic halibut transport have been parameterized for the insight and development of future studies. These results indicate that adding an additional handling step and tagging the fish improves post transfer survival. This result is clearly contrary to “common sense”. Three possible reasons for this could exist to explain this finding. A proportion of the marks on tagged fish have become faded and mark is missed during the handling of mortalities. Subsequently the fish would be enumerated as untagged fish. The second possibility is the number of untagged fish shipped was under counted/estimated and in fact more fish were shipped then the recorded 10,675. This is of course and unlikely occurrence. The third possibility is that one or more of the 11 transfer tanks of untagged fish experienced unique or
Sub-optimal conditions during transport that has lead to elevated post transfer mortality of the untagged population. A combination of all three cases is the most likely situation. In any event these biases only influence the comparison of tagged fish to the untagged population, where as the basket verses control comparison remains unbiased. Although a significant difference between basket and control groups was found, there is one problem that the experimental unit of transfer tank wasn’t properly replicated and the variation within tanks cannot be considered. For example the higher mortality observed in the control group could be attributed to a single random factor that occurred in one of the control tanks during transfer on not in one or both of the treatment tanks. Although all parameters measured during transport (water quality, temperature, dissolved oxygen and person observation) don’t indicate any marked differences between treatments and controls one must take caution when interpreting results that don’t possess adequate replication. Therefore the indication that reduced PCA within transfer tanks by utilizing wire mesh baskets will increase post transfer survival must be interpreted with caution.

Feasibility of Use:

Baskets proved more cumbersome to load and unload due to their considerable weight once loaded (~17 kg per basket) and hence were found to take marginally longer to load/unload.  To their advantage they eliminated requirement to dip net the fish at two separate points in the transfer process. The first netting is out the rearing tanks and is required for both groups.  Once loaded into treatment baskets the fish can be moved as units, eliminating the need to recapture the fish to loading them into tanks and again at the final unloading. Previous research has indicated that the act of chasing and dipping fish is the most stressful component of transport process (MAULE et al. 1988)(Robertson et al. 1988). Avoiding these steps is advantageous considering the stress response of fish is a cumulative process.

Economics:

With a commercial value of approximately $15 per juvenile even low levels of post transfer mortality can equate to a substantial economic loss to producers. The cost of the baskets used is approximately $40.00 CAN/basket and are available for purchase locally. Each transfer tank has the capacity to hold 24 individual baskets. The cost to outfit and entire live haul truck (5 tanks) would be approximately $5500 and would have and expected service life of 5 years with minimal maintenance. The overhead cost of purchasing baskets could potentially be recovered thru the mitigation of transport loss in a single load of juveniles, if the evidence observed in this trial in this study is in fact accurate. With minimal attention to design the boxes could be easily modified to be more appropriate for the loading and unloading of halibut juveniles increasing the ease and speed of handling. The use of baskets to decrease PCA within transfer tanks in combination with appropriate advances with total gas management shows promise to enable the transport of halibut juveniles at higher stocking densities, subsequently reducing the cost per unit transferred.

Summary of Conclusions:

1. There is evidence to suggest that reducing PCA within transfer tanks with comparable stocking densities will reduce post transfer mortality in halibut juveniles.
2. Wire mesh cages are and economically and logistically feasible method to stratify juvenile Atlantic halibut within transfer tanks.
3. It is evident that unexplained factors are responsible for the unusually high mortality observed following these transfers. It is suggested factors such as total gas pressure of tank water , and blood parameters of individual halibut be investigated pre, during and post transfer should be investigated in future studies in a laboratory type setting.

References

1. Brown, N., 2002. Flatfish Farming Systems in the Atlantic Region. Reviews in Fisheries Science [Rev. Fish. Sci.]. Vol. 10, (3), 403-419.
   
   
2. MAULE, A. et al., 1988. Physiological effects of collecting and transporting emigrating juvenile chinook salmon past dams on the Columbia River. Transactions of the American Fisheries Society, 117(3), 245-261.
   
   
3. Reig, L., Piedrahita, R. & Conklin, D.E., 2007. Influence of California halibut (Paralichthys californicus) on the vertical distribution of dissolved oxygen in a raceway and a circular tank at two depths. Aquacultural engineering [Aquacult. Eng.]. Vol. 36, (3), 261-271.
   
   
4. Robertson, L., Thomas, P. & Arnold, C.R., 1988. Plasma cortisol and secondary stress responses of cultured red drum (Sciaenops ocellatus) to several transportation procedures. Aquaculture (Netherlands).
   
   
5. Thedinga, J.F., Moles, A. & Fujioka, J.T., 1997. Mark retention and growth of jet-injected juvenile marine fish. Fishery Bulletin, 95(3), 629-633.