Status of Rainbow trout (Oncorhynchus mykiss-

Waldaum 1792) population after the cessation of

stocking in waterways of Horton Plains, Sri Lanka.

J. Jindasa, A.B. Kothlawala & H. M. B. K. Herath

Department of Zoology, University of Sri Jayewardenepura, Nugegoda,

Sri Lanka. *Died on 26-12-2004 in the Tsunami

Received on: 2/10/2002

Accepted on: 3/12/2003

Abstract

The rainbow trout (Oncorhynchus mykiss) was originally introduced to Sri Lanka as a sport fish in 1882, then it was called Salmo gairdneri and reported to be surviving in few numbers only in the Horton Plaints, although, the regular stocking ceased in 1989. This study is concentrated into determine the status of the present population. The study consisted of regular sampling period during April 1998 to March 1999 and occasional investigations in 2000 and 2004. The Rainbow trout, size ranging from 1.0 to 39.0 cm, mature population is just about 1000 in the Horton plaints and they exhibited size dependent distribution over different stream habitats. For breeding they migrate to first order streams where fry and fingering can be found as well. Spawning period is from late December to early March in a year when the temperature was lowest. High egg mortality between 40% and 78% was observed due to poor water quality. Average length of fish caught in each station indicated downstream increase. Surface water temperature and pH had positive relationship while dissolved oxygen content and total dissolved solid showed negative relationship with downstream direction. The slit load in water is high in higher order streams. The populations examined were devoid in headwater streams other than those of Belihul Oya.

1. Introduction

The rainbow trout (Oncorhynchus mykiss) has been one of the most extensively studied fish, yet it is not well understood (Gall and Crandall 1992). It is a very popular game fish because of its spectacular leaps from the water and its willingness to fight against the fishermen on the other end of the line. Further, the rainbow trout is probably the one of the older fish in culture (Wales, 1939). It is widely distributed in European cold countries and hence, is very popular in those countries. It is now being drawn attention as an invasive fish in Asin countries (Day 1989) where they have been introduced during the British regime.

North American trout species were originally placed in the genus Salmo. The genus includes Atlantic salmon and Eurasian species of trout. Smith and Stearley (1989) concluded that North American trout species are more closely related to Pacific salmon (Oncorhyncus) than they are to Eurasian trout species (Behnke 1992). The species name mykiss was first used to describe the rainbow trout of Kamchatka (formerly called, Salmo mykiss) by Walnaum in 1792. Bahnke (1992) described how in 1836, Richardson described a rainbow trout from the Columbia River as a Salmo gairdneri. This meant that the oldest species name for rainbow trout was mykiss. Therefore, it is called by this name.

Rainbow trout are morphologically, a typical rover-prdator. They have a fusiform, torpedo-like, body and a terminal mouth. The pelvic fins are positioned abdominally and ventrally The pectoral fins are placed low on the body, and are widely separated from the pelvic fins. Rainbow trout have an adipose fin, which is positioned just posterior of the dorsal fin. It is believed this fin might be important in the embryonic stage of development, when the other fins are not fully developed. They have a short dorsal fin, forked tail for speed, and an axillary process by the pelvic fins (Moyle and Cech 1996). The fin, for the most part, are evenly distributed over the body. This helps with stability and maneuverability while foraging in fast water.

Rainbow trout have a physostomous swim bladder, and cycloid scales form the outer covering of the fish. There are between 120-210 scales in the lateral line, depending on the subspecies and form (Behnke 1992). They possess large numbers of pyloric caeca (20-70 depending on subspecies), and have a lower number of chromosomes (58-64) than many other trout species (Behnke 1992). Rainbow trout need water with high oxygen content, because they have a low blood-oxygen affinity (about 17mm Hg)(Moyle and Cech 1996). Because of this physiological fact, rainbows flourish in the rapid moving water of streams and rivers, which tend to have higher oxygen content.

As Willers (1981) describes, trout and their closer relatives are most primitive of the bony fishes. Trout can be linked to lineages tracing back 100 million years. The Wisconsin episode, the most recent of the four Pleistocene glaciation (approx. 40,000 years ago), is thought to have crucial in the current distribution of rainbow trout. When these glaciers melted, huge lakes were left all over the current range of the western US Rainbow trout. As these slowly receded, the subspecies of trout were isolated. Rainbow were restricted south of the Columbia until the late Pleistocene glacial events. It is estimated that rainbows spread into the Columbia River drainage between 50,000 and 32,000 years ago. The Oregon populations of rainbow trout were separated from the Columbia River populations around 32,000 years ago (Behnke 1992). As the glacial lakes of the Pleistocene receded, the subspecies became more and more isolated, The California golden trout and the Sacramento red band trout are thought to be most primitive Oncorhyncus mykiss subspecies (Behnke 1992). The ultimate origin of a rainbow trout-like species was thought to be near the Gulf of Clifornia.

Oncorynchus mykiss was originally introduced to Sri Lanka as a Sport fish in 1882 (Fowke 1938, Pethiyagoda 1994, Welcome 1988), and reported to be surviving in few numbers only in the Horton Plaints although, the regular stocking ceased in 1989. (Costa 1974 and Pethiyagoda 1991 and Pethiyagoda 1994). The eyed stages of the embryos were imported from New Zealand and other countries for hatching in the Hatchery in Nuwara-Eliya. (Tissera and Tisseral 1988). Since Rainbow trout was first introduced in 1889 for game fishing, there was a well=developed sport fishery for planters until 1972 dependent on the stocked population rose by Anglers' Association of Nuwara-Eliya. The stocking with hatchery reared fingerlings has been continuing until stocking ceased in 1989. Four decades ago (1961-1969) Horton Plaints was used for seed potato cultivation from early seventies to mid seventies and due to protests from scientists and conservationists it was subsequently declared as a national reserve. In 1988 it was elevated to the status of national park and angling was banned. However, a small population sustains in the waterways of Horton Plaints, which is drained by three small mountain streams that are tributaries of three major rivers of Sri Lanka. The present study was to understand the size of the existing population with the view reintroduce angling, probable causes as to why the population does not grow and the impacts of rainbow trout on the other species of fish if there is any in Horton Plaints.

2. Materials and Methods

Study area

Rainbow trout is currently distributed only in headwater streams of Belihul Oya in the Horton Plaints national park (HPNP) in Sri Lanka (Fig. l) The Horton Plaints national park (HPNP) is situated 2200 m above sea level in the central highlands of Sri Lanka. The site is located approximately at 6''47 to 6''50N latitudes and 80o 40'' to 80'' 46'E longitudes. Its area is 3162 ha denoted by the boundaries of HPNP. The main feature of HPNP is the undulating grasslands that are interspersed with forest patch and containing meandering streams that flow along the narrow valley streams order was hased on Staler's (1957) method. Streams or reaches of Belihul Oya were classified as small (1st order streams), intermediate (2nd and 3rd order streams) or large (4th order streams) based on above method.
image1
Sampling and analytical methods.

Sampling was carried out by monthly basis from April 1998 to March 1999 and once again in 2002 July to September. Number of collection for a Station ranged from 2 (in the case of station 17 due to remoteness) to 12. Some streams section were sampled continuously from confluence to end, and others were sampled at one to six sites. However, the marked stations were sampled on regular basis. Sample sites selected to ensure representative sampling of all habitats in a stream or a section to obtain a representative sample of all size classes of rainbow trout. To support the above, multi gears were used; gill nets (stretched mesh 2.5), scoop nets (mesh 1.0mm) and hook and line, Fingerlings were sampled using scoop-nets.
All captured individuals were measured (&plusminus 1.0mm, length), marked using spaghetti tags fastened to body and returned to the point of capture. Tags with spaghetti tags fastened to body and returned to the point of capture. Tags with different color combination were used for a particular station and they were fixed beneath the dorsal fin of the fish using modified cloth tag gun. The Shnnabel (1938) method modified and described by Baily (1951) was used to estimate the population for recaptured data [, As only few specimens, 16, were allowed to kill, they preserved in 70% alcohol for studies on fecundity and eggs. Since the captured number for a sample and for site was very low all length frequencey data were pooled in to one sample. Size structure of the sample was characterized by grouping individuals in to 20mm classes. Fecundity-Length relationship was determined by regression analysis [] stogram, was used to classify individuals as juveniles, sub adults or adults. Juveniles were fish in the smallest distinct group in the sample regardless of the time of collection. The next distinctive group, sub adults were specimens between maximum size of juveniles (10.0cm) in the sample and 25.0cm. In the next distinctive group, specimens &lte25.0cm were defined as adults. Analysis of variance (ANOVA) was used to compare conditions among grouped sampled 91st,2nd, 3rd and 4th order streams).
Redds were excavated in order to identify the developed eggs. Development stages, mortality of eggs, fry survival and the frequency of redds in each group of streams were noted. Fecundity was determined accordingly. Eggs coated with sediment were recorded.

Environmental parameters such as, Dissolved Oxygen concentrations (DO). pH Total Dissolved Solids concentrations (TDS), surface water temperature, depth width, flow rate, cover and bottom structure were measured and downward correlation with number of fish associated with each corresponding stations with above feature combination was determined. Silt load in the water was measured. The number of freshwater otter Lutra lutra encountered in each of the visit and their feeding behaviour was recorded

3. Results

Nature of the streams

The topography of the occupied streams ranges from narrow, low gradient creeks to small, moderately gradient streams. Average flow ranges from <0.2ms-1 in the smallest 1st order streams (station: 1,4 and 6), to 0.1-04ms-1 in intermediate sized 2nd order streams (stations: 2,3,5 14 and 15), and to 0.12 to 0.66 ms-1 in large 3rd and 4th order streams (stations: 7,8,9,10,12,13,16, and 17). The 3rd and 4th order streams contain pools with the diameter ranging from 2m to 12m and depth from 2m to 4m. Within 1st and 2nd order streams, flow often cascades among deep (0.49-1.9m) plunge pools formed around boulders. In between pools and larger streams, channels meander and habitat varied from cobble riffle to pools formed around debris debris piles and boulders. Aquatic vegetation consists of mainly Aponogeton sp.

Population Estimation

The estimated population of O.mykiss as indicated by mark and recapture was 1010 in waterways of Horton plains. Few fish present down the Galagama Falls off Station 17.(Table 1)

Size of Fish

Decomposed length frequence distribution (Fig.2) of 290 specimens of O.mykiss using Bhattacharya's method revealed that their length ranged from 1.0 to 39.0cn with three peaks, which correspond to fingerlings (mean length -4.28 &plusminus 2.02, N=18) juveniles (mean length, 19.92 &plusminus 5.91, N &plusminus 246) and adults (mean length, 34.51 &plusminus 2.265, N=19).
