Research PaperEnriched stable isotope marking of hatchery trout via immersion: A method to monitor restocking success
Introduction
Suitable habitat for a range of salmonid species is estimated to decline by 35–80% within the next 50 years (Kennedy et al., 2009, Wenger et al., 2011, Wenger et al., 2013) which is of great concern, particularly for the survival and abundance of rainbow trout Oncorhynchus mykiss and brown trout Salmo trutta. Wild-caught and hatchery-reared forms of rainbow and brown trout have been transplanted or introduced for food or sport in at least 45 countries around the globe (MacCrimmon and Marshall, 1968, Crawford and Muir, 2008, GISD, 2017a, GISD, 2017b) and are two of the most popular recreationally fished salmonids worldwide. However, habitat range for rainbow and brown trout is predicted to constrict, as both species have a low upper thermal tolerance limit (Jonsson and Jonsson, 2009). Furthermore, altered habitat quality and availability may lead to an increased threat of parasites and disease, higher mortality rates during dry seasons, delayed (or altered) spawning, and decreased post-juvenile growth, survival, and maturity (Jonsson and Jonsson, 2009).
The continual restocking of wild fish populations with hatchery reared fish plays a major role in conserving recreational fisheries for trout (e.g. Halverson, 2008). However, the extent to which restocking enhances fisheries that include both wild and hatchery produced fish is often poorly understood, due to difficulties in accurately identifying the origin of fish caught during recapture surveys. Monitoring restocking is a crucial part of fisheries management, and it may be necessary in the future to mark or tag all hatchery produced fish pre-release for the purpose of ensuring best-practice fisheries management.
There are numerous mass marking methods that have been used for marking hatchery reared salmonids for monitoring purposes (Hammer and Blankenship, 2001), for example adipose fin clipping (Johnsen and Ugedal, 1988, Vander Haegen et al., 2005), coded wire tags (Johnson, 1990, Courtney et al., 2000), otolith thermal marking (Volk et al., 1999, Morita et al., 2013), calcein marking (Negus and Tureson, 2004, Stubbing and Moss, 2007) and to a lesser extent, alizarin complexone (Van der Walt and Faragher, 2003) and otolith dry marking (Rogatnykh et al., 2001). Of these, adipose fin clipping by itself or combined with another tag is the most widely used method to help assess trout stocking (e.g., Armstrong, 1949, Hayes and Livingstone, 1955, Baxter et al., 1988, Vander Haegen et al., 2005). However, if no automated system is available, adipose fin clipping is done by hand, meaning the process is time consuming. In addition, recent studies suggest there are possible side effects of adipose fin clipping on swimming ability (Reimchen and Temple, 2004, Buckland-Nicks et al., 2011), which may have a fitness cost for released fish.
Marking salmonids with coded wire tags can produce accurate sequential marks if detected, but the cost to mark (>$US 0.06 per fish, Hammer and Blankenship, 2001) means it is often financially unfeasible. Conversely, otolith thermal marking can be cost effective to mark, but there are problems with correctly identifying marks (Hagen et al., 1995). Calcein marking via immersion is a viable alternative marking method that is more efficient to apply than adipose fin clipping. Calcein produces internal and external green marks for salmonids (Mohler, 1997, Mohler, 2003) and has been trialled on rainbow trout in North America (Negus and Tureson, 2004, Elle et al., 2010). If applied at the appropriate concentration and exposure time, marking can be 100% effective, with little effect on mortality or growth (e.g. Mohler, 1997, Crook et al., 2009). Internal marks in the otolith are thought to be permanent, but there are ongoing issues with long term correct identification of external marks (Elle et al., 2010, Ingram et al., 2015) and the cost to mark, which is approximately $US 0.05 per fish (estimated from Crook et al., 2009 and www.chemical-reagent.com). This leaves many hatcheries without an easy to apply, low cost, accurate, welfare friendly, marking method for salmonids grown for restocking purposes.
We investigated two alternative mass marking methods for hatcheries that will enable marking of all hatchery-reared rainbow and brown trout that are restocked for enhancing recreational fisheries. The two methods, larval immersion (de Braux et al., 2014) and egg immersion (Warren-Myers et al., 2015a), mark the otoliths of fish with enriched stable isotopes and were recently modified for marking Atlantic salmon Salmo salar, but have not been tested on rainbow and brown trout. Stable isotope marking has been successful with a range of barium isotopes, which means it is capable of marking each year’s production from a hatchery with a different unique mark with 100% mark success (Woodcock et al., 2011b, Warren-Myers et al., 2015a). This would enable long term, accurate assessments of the contribution hatchery produced rainbow and brown trout make to wild fisheries. The minimum isotope concentration required and immersion time to achieve 100% mark success using immersion marking varies among fish species (e.g. 30 μg L−1 for golden perch Macquaria ambigua immersed for 1 day, Woodcock et al., 2011a; 100 μg L−1 for murray cod Maccullochella peelii immersed for 6 days, Woodcock et al., 2011b; 30 μg L−1 for Atlantic salmon immersed for 1 h, de Braux et al., 2014), which means validation and optimisation of the marking methods is necessary for rainbow and brown trout eggs and larvae before hatchery-scale use. Once optimised, both mark delivery methods can be cost effective to mark ($US 0.01–0.002 per fish, de Braux et al., 2014, Warren-Myers et al., 2015a), easy to apply and are more welfare friendly compared to fin clipping. We assessed mark success and mark strength in the otoliths of rainbow and brown trout for both delivery techniques, and outline the associated practicality of marking with stable isotopes in hatcheries.
Section snippets
Marking process
Eggs and larvae of rainbow and brown trout broodfish from the Snobs Creek Hatchery, Eildon, Victoria, were used to test the two enriched stable isotope mass marking methods, namely egg immersion (Warren-Myers et al., 2015a) and larval immersion (de Braux et al., 2014). Enriched forms of the rare isotopes 136Ba (7.8% prevalent) and 137Ba (11.2% prevalent) were selected for marking and are compared to the more abundant form 138Ba which is 71.7% prevalent. Marking via egg immersion was tested in
Survival
Immediately post-treatment there were three mortalities from a total of 545 yolk sac larvae in the larval immersion experiment, possibly due to larvae being crushed by the air stones placed into tanks to aerate the isotope solution during the 24 h immersion period. After immersion, no further larvae died during the 4-week grow out period. In the egg immersion experiment, the percentage survival to eyed stage was >80%, which is typical for rainbow and brown trout (Bromage et al., 1992).
Mark success
For the
Hatchery marking
Ideally, all hatchery fish should be marked to enable later identification and proper monitoring of restocking efforts. However, for many hatcheries, the cost, difficulty in applying a method, or the inability to identify marks with high confidence post release, means marking is not standard hatchery practice. Here, we have demonstrated one of the methods previously validated, larval immersion (de Braux et al., 2014), can eliminate these issues for two widely stocked salmonid species. The
Acknowledgements
This study was funded through the Victorian Government to improve recreational fishing in Victoria through revenue from Recreational Fishing Licences, with support from Fisheries Victoria and the University of Melbourne. The authors wish to thank staff at the Snobs Creek Hatchery, Eildon, particularly Hui King Ho, Neil Hyatt and Stephen Vidler, for their assistance during the study.
References (45)
- et al.
Origin of juvenile Pacific salmon recovered from coastal southeastern Alaska identified by otolith thermal marks and coded wire tags
Fish. Res.
(2000) - et al.
Comparison of growth rate in Atlantic salmon, pink salmon, Arctic char, sea trout and rainbow trout under Norwegian farming conditions
Aquaculture
(1978) - et al.
The effects of weathering on rare-earth element, Y and Ba abundances in Tertiary basalts from southeastern Australia
Chem. Geol.
(1991) - et al.
Otolith thermal marking
Fish. Res.
(1999) Mortality, rate of growth, and fin regeneration of marked and unmarked lake trout fingerlings at the provincial fish hatchery Port Arthur, Ontario
Trans. Am. Fish. Soc.
(1949)- et al.
Blue Rock Lake: an example of how fin-clipped trout have helped to manage the trout fishery
Fisheries Management Report No. 23
(1988) - et al.
Broodstock management, fecundity, egg quality and the timing of egg production in the rainbow trout (Oncorhynchus mykiss)
Aquaculture
(1992) - et al.
Neural network detected in a presumed vestigial trait: ultrastructure of the salmonid adipose fin
Proc. R. Soc. B
(2011) - et al.
Global introductions of salmon and trout in the genus Oncorhynchus: 1870–2007
Rev. Fish. Biol. Fish.
(2008) - et al.
Development and evaluation of methods for osmotic induction marking of golden perch Macquaria ambigua with calcein and alizarin red S
N. Am. J. Fish.
(2009)