IDDT

Introduction

Infestation of salmon lice (Lepeophtheirus salmonis) represents a serious challenge to the industry in the leading producer countries of Atlantic salmon (Salmo salar), such as Norway, Scotland and Canada. Another ectoparasite, Caligus elongatus, is less host-specific than salmon lice but can cause harmful effects to salmon and other fish species. Along with the increasing salmon production the occurrence of the ectoparasite salmon lice has expanded dramatically [1] and induces major costs and fish welfare problems for the industry [2] and, not least, poses a threat to stocks of wild salmon and sea trout [3,4]. Sea lice feeding may cause skin lesions of their hosts leading to secondary infestations and osmotic failures affecting growth, fertility, and survival [5].

Over the years, different approaches for controlling lice infestation in cage farming have been introduced, which can roughly be divided into three groups according to Jónsdóttir et al. [6]: immediate (chemotherapeutic treatment), continuous (mechanical, thermal and freshwater treatment, deployed cleaner fish), and preventative measures including physical barriers (lice skirts, submerged ‘snorkel’ cages).

Immediate treatment, use of medical drugs, has been the dominating measure to combat sea lice since the 1970s [7]; however, the growing concern about emerging treatment-resistant lice at cage farms [8] and demonstrated harmful effects on the local environment [9] has led to implantation of alternative treatment methods. Handling of fish during repeated mechanical – thermal – freshwater treatment causes stress and risk of injuries to the fish [10] and is considered to contribute to mortality and reduced growth during the cage-based grow-out cycle [e.g. 11].

Barriers, such as impermeable and semi-permeable skirts mounted floating cages, or use of submersed cages (‘snorkel’ cages) kept below the upper layers where infectious sea lice are most concentrated, have demonstrated substantially reduced lice infestation [7,12]. Preventative measures also include stocking of ‘cleaner fish’ which are feeding on lice attached to salmon often combined with physical barriers [13]. Of all sea lice management measures in Scotland, use of skirts was ranked the most costeffective measure with under £0.10 per fish per unit of effectiveness [14].

In several recent reviews, Jónsdóttir et al. [6], Oppedal et al. [15] and Barrett et al. [7] presented comprehensive surveys of efficiency and environmental challenges of lice shielding skirts and other lice-preventive barriers in salmon cages. This paper shortly describes use of lice skirts mounted floating cages emphasizing the lice reducing effect under different conditions based on literature.

Salmon lice

The ectoparasite has eight life stages: the first three are freeswimming (nauplii and copepodid stages), the next two are attached to the host (chalimus I-IV stages) and the final three are mobile (pre-adult and adult stages), Figure 1. Copepodids, the infective stage, are more actively moving than the preceding nauplii to detect and infest a host [16] and the swimming behaviour and infestation success are connected to environmental conditions, such as light, current velocity, salinity and temperature. Many factors, among others diurnal vertical movement pattern and the salinity profile, will affect the position of the lice larvae; however, it has been suggested that they mostly remain in the uppermost 4 m of the water surface [17]. The free-swimming larvae are small, between 0.4 and 0.7 mm long [5]. A detailed survey of sea lice ecology is presented by [16].

Figure 1: The eight life stages of salmon louse (Lepeophtheirus salmonis) [18].

Lice skirts

Lice skirts are either impermeable or semi-permeable plankton nets. The materials utilised as the barrier can be impermeable membranes (using canvas- or tarpaulin-like material) or semi-permeable to filtering out copepodids [15]. A standard WP-2 plankton net with 200 μm mesh size will collect free-swimming sea lice larvae satisfactorily [19]. As an example, skirts with a mesh size of 350 x 350 µm have been applied at Norwegian cage farms [6]. Skirts have wide application and were estimated used at more than 300 cages annually some years ago [15].

Figure 2: A 10 m deep lice-shielding skirt in a circular 120 m circumference cage (design: Svein B. Tveiten).