Salmo trutta
Written by Jason Tickner (Lincoln University)
July 2005
Edited by Michael Hudson, Lincoln University, June 2006
Taxonomy
- Scientific name: Salmo trutta (Linnaeus,1758)
- Popular Common names: Brown Trout
- Kingdom: Animalia
- Phylum: Chordata
- Class: Osteichthyes
- Order: Salmoniformes
- Family:Salmonidae
- New Zealand Status: naturalised
- Taxonomy sources: Microsoft Encarta 2005
Identification
Species Description
The Brown trout (Salmo trutta) is ‘probably the most important recreational fish in New Zealand’ (McDowall, B. 1984, p.18) making it well known to many outdoorsmen. The most distinctive characteristic on this slender, streamlined fish is the black and sometimes reddish spots which cover most of its body. In lake, river and sea environments a trout’s colouration varies to blend with each individual habitat (figure 1). Lake-fish typically are of brownish-olive colour while river-fish are similar but with a golden tinge across the body. Sea-run fish are primarily silver with unmistakable dots. All brown trout have large mouths, especially males, extending below the eyes and adipose fin (the top fin near the tail) is relatively well-developed. In the right conditions brown trout can reach 1400mm in length and weigh over 20kg. Usually most fish will only reach 800mm and round 5kg (McDowall, 2000).
Figure 1. Brown trout of lake, river and sea environments (image source: McDowall, 2000, p.114)
Similar species
There is one other common species of trout that can be found in New Zealand’s rivers. The rainbow trout (Onorhynchus mykiss) is found in most rivers and dominants inland lakes. A bit smaller than the brown trout, 2–3kg are routine, the river-run rainbow trout displays a beautiful plume of colour running from green, pink and silver across its body along with dark speckles as seen in figure 2. The lake-fish of this species is similar but with a silver dominated colour in place of the green which the river-fish uses as camouflage (McDowall, 2000). Another distinctive feature, which helps distinguish between these two species, is the tail of brown trout lacks the rows of spots seen on the Rainbow trout’s tail (McDowall, 1984).
Figure 2. Rainbow trout of both lake and river environments (image source: McDowall, R. 2000, p.120)
The Brown trout is also easily confused with the Atlantic salmon (Salmo salar), its closest relative. These species only populate the upper lakes of Waiau River in Southland, thereby saving confusion in other regions. The Atlantic salmon displays a silver belly and dark blue to green back on a shallower frame compared to the Brown trout (figure 3) (McDowall, 2000).
Figure 3. Atlantic salmon (image source: McDowall, R. 2000, p.117)
Most confusion between species occurs during the juvenile stage when these characteristics are not vivid and colouration may vary.
Geographic Distribution
Full natural and naturalised range
Brown trout, being an introduced species, originate from a separate continent. Their natural range spans from Turkey across to West Europe (McDowall, 2000). Introduced during the 1860’s they, and other species, have become naturalised to the New Zealand environment (McDowall, 1984). Currently introduced to 24 countries outside Europe, this fish has become a global species as shown in figure 5 (Elliott, 1994).
Figure 5. World distribution of Brown trout (image source: Elliott, 1994, p.12).
New Zealand range
Brown trout are wide spread covering practically the whole country where sufficient waterways are found. The only areas where they are not found in New Zealand are Stewart Island and the Chatham Islands (McDowall, 2000).
Canterbury range
Most natural water ways with sufficient depth contain brown trout. Also found in lakes, estuaries and in the sea surrounding the coast, brown trout have an expansive range across Canterbury.
Natural History in Canterbury
Habitats
From alpine-lakes and fast-flowing headwaters to estuaries and tidal lakes, the brown trout are found in most bodies of water (McDowall, 2000). In a river system trout prefer sitting in deep pools or, when feeding, often sitting behind a large rock or bolder while waiting for food to come to them (personal observation).
The major determining factor of brown trout habitat is the water temperature. A temperature between 4 to 19.5 degrees centigrade is needed for Brown trout to thrive (Elliott, 1994). Well oxygenated water is also a key requirement for their survival (McDowall, 2000).
Preferred habitats
Cold, flowing, well oxygenated environments are most preferred by Brown trout. A study preformed by two Otago University Zoologists found that:
“Brown trout (mean fork length 42 cm) preferred a mean depth of 65.0 cm and a mean velocity of 26.7 cm second-1 at the position occupied by the fish for feeding, but for spawning they preferred a mean depth of 31.7 cm, a mean velocity of 39.4 cm second-1, and a mean substrate size of 14.0 mm.” (Shirvell & Dungey, 1983, p.355)
Phenology/Breeding
Brown trout spawn in autumn or early winter. Firstly the female finds a suitable gravel bed and creates a depression in the shingle where she then lays her eggs. At the same time several males will contest to see who will fertilise the eggs. This successful dominant male then sheds his sperm and the female will begin this cycle again until all her eggs are fertilised (Elliott, 1994). Egg clutches may range from one hundred to over two thousand. A healthy female brown trout will spawn annually for eight to ten years (McDowall, 2000).
Life cycle
A brown trout’s life starts from the moment it hatches from an egg in a gravelly nest. It takes form as alevin; a hatchling feeding from a yolk-sac until it reaches the next stage of its life cycle. The fry stage is a short transition between alevin and parr where the trout emerges from its shingle habitat and begins to disperse and feed. Parr is the next stage and the yolk-sac is fully absorbed (Elliott, 1994). After this the trout will continue to grow until ready to spawn; and so the cycle continues (figure 6).
Figure 6. The life or production cycle of the trout (Image source: www.spaquaculture.com/default.aspx?pageid=505)
Longevity
As stated by McDowall (1984), brown trout are long-lived with some fish known to live up to 12 years. The key to their longevity is their ability to survive spawning. Unlike their close relatives the Atlantic salmon and Rainbow trout, brown trout continues growing after they spawn (McDowall, 1984).
Diet
Brown trout are not specific feeders when it comes to where they dine. They will feed from the river bed to the water surface thereby allowing access to a wide range of foods throughout the year. Although their diet is varied, brown trout often become very selective towards certain foods during different times of the year. Many fishermen have come across this; presenting several different flys or lures to a trout before it strikes. This diet includes Caddis (Trichoptera) larva, mayflies (Ephemeroptera), beetles (Coleoptera), crayfish (Paranethrops planiforns and P. zealandicus), worms (Oligochaeta), whitebait, juvenile trout (eg Salmo trutta), frogs and they are even known to eat mice and ducklings (McDowall, 1984).
Predators
Brown trout have two main predators in New Zealand. These are the shag (Phalocrocorax spp.) and the eel (Anguilla spp.). Only feeding on the smaller fish, it is said that the impact on the trout is in some ways beneficial. This is because the reduction in population densities results in more food resources for the surviving trout; in turn allowing for bigger fish and weaning out the unhealthy ones (McDowall, 1984).
Parasites
There are several different parasites which effect wild trout; but these are very uncommon and cause limited injury. When these fish are farmed in large densities parasites can have an adverse effect on the fish population. Close monitoring must be carried out to prevent this happening (Roberts & Shepherd, 1974).
How to find a Brown Trout
Brown trout have adapted to blend into their habitat. Sea, river and lake run trout are all extremely hard to spot if stationary in their natural environment. The key is to know where to look. Polarising sun glasses help to reduce glare from the water yet are not vital in finding trout. Sea and lake run trout are harder to find than river trout because their habitat means that they are only seen when they rise to feed or are in the shallows. River brown trout lose this advantage because their environment is relatively shallow. The easiest place to spot a brown trout is in slower moving pools. This is for two reasons: Firstly brown trout like to rest and feed in these areas; secondly the water is not affected by disturbance from rapids making it very clear (personal observation). Another common place to find a brown trout is behind a large rock or bolder while feeding in the rapids; but they’re much harder to view in this environment. Caution must be taken when stalking trout. They can both hear and see you, so stalk them as you would a rabbit because if disturbed, you may not see that trout again (Forrester, 1979).
The best time to view them is early morning or late afternoon. This is when the fish are most active as they feed. It is recommended to ask local fishermen for advice on when and where to look as brown trout behaviour varies in different regions (Forrester, 1979).
Abundance and Conservation Status
- New Zealand: abundant
- Canterbury: abundant
Threats
There are seven chief threats to the brown trout (adapted from McDowall, 1984):
- Pollution
- Eutrophication
- Wetland drainage
- Forest clearance
- Impoundment
- Channelisation
- Water abstraction
All of these are high priority issues throughout New Zealand and need to be dealt with in the very near future if this industry is to keep flourishing. In Canterbury, eutrophication and water abstraction are the key issues. Excessive amounts of intensive farming occur on the plains, these practices are posing a huge threat to the local fisheries. Poisonous algal blooms from tonnes of fertiliser and effluent entering these waterways make habitats like Lake Ellesmere uninhabitable. In addition the Canterbury plains are in the rain shadow of the Southern Alps resulting in irrigation being an important tool for efficient farming. The resultant consequences for trout in the area are dire with rivers, such as the Selwyn, drying out repeatedly in the summer (McDowall, 1984).
Introduced species that thrive in our waters have had adverse affects on our native freshwater fish.
‘Trout have been linked with the reduction in both the range and population size of a number of native species’ (Jellyman & Grynoth, 1994, p.8).
But these effects, unlike other introduced species, are not near as threatening.
Significance for people
Brown and Rainbow trout form the base for the majority of New Zealand’s freshwater recreational fisheries. The economic significance through fisheries and support industries is hard to determine but to get a rough idea, Taupo fisheries expenditure was assessed to be $16.7 million for the period 1982–83 (Jellyman & Grynoth, 1994).
Historically trout were introduced for sport and made illegal to sell. Now, through guided fishing trips, people can make a living off this naturalised resource. Even more important than the economic value is the pleasure every New Zealander has knowing that he or she, regardless of age, can pick up a rod and enjoy this exciting sport in our country’s beautiful surroundings.
Summary
The Brown trout has acclimatised to New Zealand’s environment and can be seen in most of our streams, rivers and lakes. This fish has provided a source of recreation for thousands of people; both locals and international visitors. It also provides economic benefits to the country estimated at $150 million (Jellyman & Grynoth, 1994). To ensure this resource is retained for future generations, emphasis must be put on the management practices of the land surrounding their habitats and regulations need to be abided by. We live in a beautiful country; so take advantage of it and try your hand at catching one of New Zealand’s most prized fish in the world’s most picturesque landscape.
References
Elliott, J. M. (1994). Quantitative Ecology And The Brown Trout. New York, United States: Oxford University Press Inc.
Forrester, R. (1979). Trout Fishing in New Zealand. Christchurch, New Zealand: Whitcoulls Publishers.
Jellyman, D. J. & Graynoth, E. (1994). Headwater Trout Fisheries In New Zealand. Christchurch, New Zealand: NIWA Freshwater.
McDowall, B. (1984). Trout in New Zealand Waters. Wellington, New Zealand: The Wetland Press.
McDowall, R. M. (2000). New Zealand Freshwater Fishes. Auckland, New Zealand: Reed Books.
Roberts, R. J. & Shepherd, C. J. (1974). Handbook of Trout and Salmon Diseases. Surrey, England: Fishing News (Books) Ltd.
Spackman, W. H. (1892). Trout in New Zealand. Wellington, New Zealand: George Didsbury, Government Printer.
Stokell, G. (1955). Fresh Water Fishes of New Zealand. Christchurch, New Zealand: Simpson and Williams Ltd.
Web based resources
Search for Salmo trutta in New Zealand science online
New Zealand Journal of Ecology articles containing Salmo trutta.
Search NZ Royal Society journals for articles containing Salmo trutta.
