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1 Aquaculture 312 (211) Contents lists available at ScienceDirect Aquaculture journal homepage: Population dynamic of early stages of Caligus rogercresseyi in an embayment used for intensive salmon farms in Chilean inland seas Carlos Molinet a,b,c,, Mario Cáceres d, Maria Teresa Gonzalez f, Juan Carvajal e, Gladys Asencio e, Manuel Díaz a, Patricio Díaz a, Maria Teresa Castro e, José Codjambassis a a Instituto de Acuicultura, Universidad Austral de Chile, Los Pinos s/n, Balneario Pelluco, Puerto Montt, Chile b Centro Trapananda, Universidad Austral de Chile, Portales 73 Coyhaique, Chile c CIEN Austral, Los Pimientos 4369, Puerto Montt, Chile d Universidad de Valparaíso, Borgoño 16344, Montemar, Viña del Mar, Chile e Centro I-mar, Universidad de Los Lagos, Camino a Chinquihue Km 6, Puerto Montt, Chile f Instituto de Investigaciones Oceanólogicas, Universidad de Antofagasta, Casilla 17, Antofagasta, Chile article info abstract Article history: Received 28 June 21 Received in revised form 5 December 21 Accepted 7 December 21 Available online 13 December 21 Keywords: Sealouse Pest management Larvae Ovigerous female Circulation pattern Salmon farm Around the world several strategies for the management and control of sea lice infestations have been implemented. In Chile where the salmon harvest is significantly higher than other countries and where salmonids are not endemic species, information concerning how farm locations and local oceanographic conditions interact in regulating the dispersal of parasites and their impact on farmed and native fish populations is required urgently. In this work we studied the spatial and temporal dynamics of the early stages of Caligus rogercresseyi in a semi-enclosed area where eight salmon farms are located in southern Chile. Plankton samples were collected in three modes (fortnightly, diurnal and semidiurnal spatial), which were complemented by studying circulation patterns inside the bay. Simultaneously, records of ovigerous female C. rogercresseyi per fish, treatments for caligids, biomass, and the average weight of fish were obtained from salmon farms in the study area. Our results suggest that the population dynamics of the early stages of C. rogercresseyi is strongly associated to salmon farm, which could be magnified by the effect of the local circulation pattern. In this context it seems unlikely to control, through current caligid treatments, the caligid pest in this semi-enclosed area without a drastic decrease of salmon biomass inside the bay. The salmon farms should be located distant enough between them to minimize risks and maximizes benefits to all concerned parties. 21 Elsevier B.V. All rights reserved. 1. Introduction Around the world several management strategies for the control of sea lice infestations have been implemented (Andersen and Kvenseth, 2; Bron et al., 1993; Costello, 29; Kvenseth and Kvenseth, 2). According to Revie et al. (29) when large numbers of farmed salmon are introduced to the marine environment within open net cage salmon farms, it is virtually to impossible to avoid (1) the infection of farmed fish, all of which go into the pens as clean smolts, and (2) the subsequent infection of wild fish that are found in the vicinity ( infective field ) of the installation. Therefore, management strategies must be designed taking into account the regional, biological and environmental factors that influence the severity of sea lice infections (Brooks, 29; Murray and Peeler, 25; Penston and Davies, 29; Revie et al., 29). Corresponding author. Instituto de Acuicultura, Universidad Austral de Chile, Los Pinos s/n, Balneario Pelluco, Puerto Montt, Chile. Tel.: ; fax: address: cmolinet@uach.cl (C. Molinet). In Chile where the salmon harvest is orders of magnitude higher than in other countries, and where salmonids are not endemic species, urgent attention is required in order to understand how farm location and water current patterns interact in regulating the dispersal of parasites, and their population dynamics (Buschmann et al., 29). Caligus rogercresseyi Boxshall and Bravo (2) is the dominant sea louse parasite affecting the salmon and trout industry in Southern Chile (González and Carvajal, 23). The parasite was transmitted to the farmed fish by the native rock cod Eleginops maclovinus and Odonthestes regia (Carvajal et al., 1998). These were the most common wild hosts of C. rogercresseyi, C. teres, C. cheilodactylus and Lepeophtheirus mugiloides (Carvajal et al., 1998; Revie et al., 29). Despite the impact of the caligid parasite on the salmon industry in Chile (Bravo, 23; Johnson et al., 24) and the likelihood that transmission between populations occurs during the planktonic stages, there is no published account of the larval ecology of C. rogercresseyi. Also, increasing economic losses due to parasite infestations have led to the increased use of chemicals to control the caligids (Bravo et al., 28; Pino-Marambio et al., 27). Laboratory studies have shown that C. rogercresseyi has eight developmental stages: three planktonic and five parasitic (González /$ see front matter 21 Elsevier B.V. All rights reserved. doi:1.116/j.aquaculture

2 C. Molinet et al. / Aquaculture 312 (211) and Carvajal, 23). The planktonic stages are non-feeding and comprise two nauplii and one copepodid stage. It is during the latter stage that infection occurs. The planktonic stages begin with the first nauplius (average length=425 μm), 3 days later the larvae transforms into the second nauplius (average length=463 μm). Once transformed into the copepodid stage it settles on the host, holding on with its hooked pair of antennae (González, 26). Depending on seawater temperature, as observed in Lepeophtheirus salmonis (Stien et al., 25), the lifespan of the planktonic stages vary, and can last between 5 and 9 days in the laboratory, after which the copepodids are ready to settle (Farias, 25; González, 26). The parasitic stage begins with first chalimus, develops through to the fourth chalimus, finally ending in the adult males and females. Attachment is stronger after the fifth day when the accumulated temperature is over the 5th degree days of effective temperature (González and Carvajal, 23). During the 5 to 9 days spent in the plankton, significant transport and dispersion within surface currents are possible. Over this period of time, the transmission risks to other populations will depend on the rate of dispersion and the direction of transport from the point of origin, as has been observed for the larvae of the sea louse of L. salmonis (Kroger, 1838; Amundrud and Murray, 29). To estimate the transmission potential of sea lice, it is essential to understand where the local circulation may transport the planktonic stages of the lice. Epidemiological studies have found that L. salmonis abundance on fish farms is related to local current speeds and the sea loch flushing times, highlighting the importance of the physical circulation (among other factors) to the infection processes (Amundrud and Murray, 29; Genna et al., 25). Planktonic stages of sea lice are vulnerable to low salinities (Stien et al., 25) and have been described as essentially passive, with a limited vertical swimming ability that allows them to remain near the surface (Bron et al., 1993). Also, laboratory experiments have shown that the copepodids of L. salmonis are strongly phototactic (Genna et al., 25), confirming the field observations of Heuch et al. (1995). These authors observed that nauplii exhibited only small differences in depth between night and day while copepodid distribution appeared to be controlled by light intensity. These results suggest the presence of aggregations of larvae in surface layers during daylight; however, despite intensive sampling along coasts of several countries, pelagic swarms of copepodids have not been found (Penston et al., 22). The objective of this research was to study the spatial and temporal dynamics of the early stages of C. rogercresseyi associated with the abundance of C. rogercresseyi ovigerous females in a semienclosed area where 8 farms are located in southern Chile, in order to provide a more comprehensive source of information for developing management strategies for salmon farming. 2. Methodology 2.1. Study area The study was carried out in Codihue Bay (41 48 S and 73 2 W) at the northwestern end of Patagonian inland sea. Codihue Bay has an east west orientation, a maximum depth of 6 m, and opens to the east. The bay is ~12 km long, and 3 4km wide, and is connected to Chacao Channel through the Abtao Channel (Fig. 1). The water column is relatively homogeneous (around 32 psu) and temperatures range between 9 C and 15 C at the surface during winter and summer, respectively. Eight salmon farms were operating during the period of the present study (named F to F7); F being an experimental farm (Fig. 1) Data collection In order to study larval spatial and temporal variability in C. rogercresseyi within the study area and their relationship with environmental variables, three modes of plankton sampling were carried out: i) fortnightly, ii) diurnal, and iii) spatial-semidiurnal, which were complemented with temperature and salinity records, and a characterization of sea current patterns. C. rogercresseyi larvae were sampled by towing plankton nets (.6 m in diameter 1.5 m in length; with 15 μm mesh size) from a 1 15-meter boat. The towed distance was estimated from the boat Fig. 1. A) Region of South America (black arrow) where the study took place in Southern Chile (Chile is highlighted in dark colour). B) Chilean inland sea in northwest Patagonia, the position of Codihue bay is denoted by a rectangle. C) The fine scale representation shows the, 1, 2, 3, 4, 5, and 6-meter isobaths (gray dotted lines). Also displayed are the sample stations of mode 1 (annual) (St 1, inside the bay, and St 2 outside the bay); the track followed during the 24 h of ADCP sample collection (dark gray dotted which formed an irregular polygon) and the four transect studied (T1 = transect 1, T2 = transect 2, T3 = transect 3, and T4 = transect 4). Black dots at the ends of transect 2 shows the CTD east and west stations. Crosses show the nine sample stations of mode 3 (spatial semidiurnal). Gray rectangles show the location of farms which producing salmon during the study period.

3 64 C. Molinet et al. / Aquaculture 312 (211) velocity (2 knots, controlled by a GPS Garmin), and time of tow, which ranged from 3 to 5 min depending on the sample mode Mode 1 Fortnightly samples were collected from July 28 to June 29 at one inside station and one outside station of Codihue Bay (Stations 1 and 2 in Fig. 1). Within two strata (surface and 1 15-meter depth) three samples were taken by towing plankton nets for 5 min at a speed of ~2 knots. Over the same period, ovigerous females of C. rogercresseyi were sampled from netpen-reared rainbow trout (Oncorhynchus mykiss) and Atlantic salmon (Salmo salar) (1 fish per cage) from eight farms in Codihue Bay. In addition, monthly values of salmon biomass from the same farms were obtained. Using the averaged weight of fishes sampled and the total biomass we estimated the number of fishes. Larval abundance Larval abundance Larval abundance Larval abundance A) B) C) D) Aug-8 Aug-8 Aug-8 Sep-8 Sep-8 Sep-8 Oct-8 Nov-8 Jan-9 Feb-9 Date (Month-year) NI NII COP Oct-8 Nov-8 Jan-9 Feb-9 Date (Month-year) NI NII COP Oct-8 Nov-8 Jan-9 Feb-9 Date (Month-year) NI NII COP Surface, Station 1 May-9 Jun-9 15 m, Station 1 May-9 Jun-9 Surface, Station 2 May-9 15 m, Station 2 Aug-8 Sep-8 Oct-8 Nov-8 Jan-9 Feb-9 May-9 Date (Month-year) NI NII COP Fig. 2. Caligus rogercresseyi larval abundance (nauplii II and copepodids) inside Codihue bay (at the surface (A) and 15-meter depth (B)); and outside Codihue bay (at the surface (C) and 15-meter depth (D)).

4 C. Molinet et al. / Aquaculture 312 (211) Mode 2 Diurnal sampling (24 h) was undertaken on July 29/3, 28 (winter survey); April 24/25, 29 (autumn survey); and November 16/17, 29 (spring survey). Three samples were collected simultaneously at the surface, 1 15 m, and 2 25 m by plankton nets towed for 5 min, every h at station 1, inside Codihue Bay (Fig. 1C). Temperature and salinity were recorded at the beginning and end of the tows with a Seabird SBE-19 conductivity temperature depth (CTD) recorder. During the autumn survey, current velocity profiles were recorded with a broadband 3 khz RD-Instruments acoustic Doppler current profiler(adcp)across four transects: 1) eastern (across-bay), 2) southern (along-bay), 3) western (across-bay), and 4) northern (along-bay (Fig. 2A)). These transects were sampled 14 times during a 24-hour period. The ADCP was towed on a 1.2- meter long catamaran positioned on the starboard side of the R/V Monica VI at speeds of between 2. and 2.5 m s 1.Velocityprofiles with a vertical resolution of 2 m and ping rates of ~1 Hz were averaged every 6 s, yielding a spatial resolution of m. The ADCP compass was calibrated using Global Positioning System (GPS) navigation data Mode 3 Spatial sampling (a semidiurnal tidal cycle) was undertaken on December 4th 28, using a spatial grid of nine stations inside Codihue Table 1 Results from the sequential deviance analysis for the response variable ovigerous female of C. rogercresseyi between May 28 and June 29. Predictive variables were: month, farm, total biomass and total number of fish in salmon farms of Codihue bay, host species, fish weight and chemical treatment for caligids. Degrees of freedom Deviance Df. Deviance P (Nχ 2 ) Null Month Farm Biomass Host species Mean fish weight Number of fishes Treatment Bay where two plankton nets were towed for a period of 3 min at 2 knots at each station. Surface samples were collected every 3 h and temperature and salinity were recorded in a DST logger attached to each net. In addition, before the plankton sampling began, ten drifters were deployed around station 1 (Fig. 1C) at the surface (five drifters) and at a depth of 2 m (five drifters). The movement of these drifters was then followed over the 12 h of the semidiurnal cycle. A) Biomass (Tons.) Thousands of fishes Thousands of Ovigerous female /May 28/Jul 28/Aug 28/Oct 28/Dec 29/Feb Fish weigth (gr) 29/Apr 29/Jun 29/Aug 29/Oct 29/Dec 21/Feb 21/Apr 21/Jun Biomass Nº of Fishes Nº Ov. Female Fish weight B) 4 2 Larvae/ 87 m /Jul 28/Aug 28/Sep 28/Oct 28/Nov 28/Dec 29/Jan 29/Feb 29/Mar 29/Apr 29/May 29/Jun 29/Jul 2 Thousands of Ovigerous female Date (month) COP NII Ov. Female Fig. 3. A) Estimated ovigerous female of C. rogercresseyi between May 28 and June 21 and the variation of salmon biomass, number of fish, host species, fish weight and chemical treatment for caligids at the eight farms in the study period. B) Variation of estimated ovigerous female and early stages of C. rogercresseyi during the study period.

5 66 C. Molinet et al. / Aquaculture 312 (211) Table 2 Results from the sequential deviance analysis for the response variable larval abundance of C. rogercresseyi in mode 1 of sample collection (fortnightly). Predictive variables were: strata, ovigerous female, and season Data analysis Degrees of freedom Deviance Df. Deviance P (Nχ 2 ) Null copepodid Strata Ovigerous female Season Null nauplii II Strata Ovigerous female Season To select explanatory variables for abundance of C. rogercressseyi, nauplii and copepodids during our three sample modes of data collection and for abundance of ovigerous female in the study area we applied generalized linear models, GLM (McCullagh and Nelder, 1989), assuming a Poisson distribution for errors of the untransformed response variable. To avoid collinearity among the variables, they were tested one at a time, choosing the one providing the best fitaccordingto Akaike's information criterion (AIC). A deviance analysis was used to evaluate the relative contribution of each variable in explaining the variability in the response variable. The significance of each explanatory variable was determined using a χ 2 test (Venables and Ripley, 1998). Larval abundance of C. rogercresseyi and density of the surface seawater obtained from the 9 stations of Mode 3, and track drifters were referenced and deployed in a Geographic Information System (SIG) utilizing Arc Gis 9.3. CTD data obtained during Modes 2 and 3 were processed using the manufacturer's software, and the processed data were used to produce density profiles using Surfer 7. software (Golden Software). Current velocity data were processed following the approach of Valle-Levinson and Atkinson (1999). After the heading correction was applied, the data were rotated anti-clockwise to an along- (v flow) and across- (u flow) channel coordinate system. These angles were oriented in the direction of greatest variability in the tidal currents and of weakest across-channel tidal flows. Semidiurnal (M 2 ) and diurnal (K 1 ) constituents were separated from the subtidal signal of the observed flow components using sinusoidal least squares regression analysis (Lwiza et al., 1991). Depth (m) Copepodid, winter Copepodid, autumn Copepodid, spring Depth (m) Nauplii II, winter Nauplii II, autumn Nauplii II, spring Depth (m) 3 Density (Kg/m ), winter 3 Density (Kg/m ), autumn 3 Density (Kg/m ), spring Height tide Day of August 28 to.1.2 to to to to 16 Day of April 29 Day of November 29 Fig. 4. Results of the three diurnal samples (mode 2) carried out during winter 28, autumn and spring 29. Gray zones show dark (night) hours. The first three panels show copepodids abundance during the 24-hour sampling. The second three panels show nauplii II abundance during the 24-hour sampling. The third three panels show density of the water column during the sample period. The three waves showed at the base of the panels show tide variability during the sample period.

6 C. Molinet et al. / Aquaculture 312 (211) Table 3 Results from the sequential deviance analysis for the response variable larval abundance of C. rogercresseyi in mode 2 of sample collection (diurnal). Predictive variables were: season, day period, tide, strata, and density water. 3. Results Degrees of freedom 3.1. Mode 1: annual variation Deviance Df. Deviance P (Nχ 2 ) Null copepodid Season Day period Tide Strata Density Null nauplii II Season E 14 Day period E 7 Tide E 5 Strata Density The early stages of C. rogercresseyi were more frequent inside than outside of Codihue bay with the largest abundance of copepodids (9 copepodids per 87 m 3 ) occurring during the summer months at the surface (Fig. 2A). Three apparent peaks of larval abundances were observed inside the bay (during winter, spring and summer) in subsurface samples where nauplii II and copepodids exhibited similar magnitudes of abundance (Fig. 2B). The abundance of nauplii II ranged from 1 to 1 larvae 87 m 3, while nauplii I were rare during the entire study period in all of sampling modes. All the early stages of C. rogercresseyi were scarce at the station outside the bay throughout the study period, although it was observed higher larval abundance at the surface, (Fig. 2C) than at the subsurface samples (Fig. 2D). Intensive salmon farming production occurred between April 28 and March 29 at the eight farms within the study area, reaching around 18, t of salmon (S. salar and O. mykiss), (around 6 million of fish) (Fig. 3A). Between March 29 and July 29 only O. mykiss was farmed within the study area, with an average production of 1 t. per month. Taking into account the biomass of salmon in the farms, the abundance of ovigerous females inside the bay was estimated to vary from 2 individuals in autumn 29 up to 18 million individuals during spring summer 29 (Fig. 3A). The period of maximum ovigerous female abundance was coincident with maximum abundance of copepodids, but not with nauplii II abundances (Fig. 3B). According to the GLM analysis, month (June 28, November 28, and May 29), treatment (no treatment and deltamethrin), and farm (Farm F, F3 and F4) were the most important explanatory variables of ovigerous female abundances (explaining 16%, 13% and 9% of total deviance, respectively), while farmed species and total biomass explained less than 1% of total variability. Individual mean fish weight and the total number of farmed fish in the bay did not contribute to the explanation of the ovigerous females' variability (pn.5, Table 1). Annual variability of copepodids was significantly explained by the number of ovigerous females (4% of deviance), depth strata (12% of deviance) and season (1% of deviance). Annual variability of nauplii II was also explained by the same variables, however these accounted for only 25% of the total deviance (Table 2) Mode 2: diurnal variation During the three diurnal surveys carried out between 28 and 29 only nauplii II and copepodid stages were found. As observed during Mode 1, copepodids were more frequent than stage II nauplii. Fig. 5. Vertical contours of the flow in transects 2 and 4 for both U and V components; U component are positive (yellow) toward the east, and negative (blue) toward the west. Component V is positive (yellow) toward the north, and negative (blue) toward the south. White band close to the bottom shows data not considered for analysis because of their interaction with bottom echoes.

7 68 C. Molinet et al. / Aquaculture 312 (211) Nauplii II were more abundant during the winter survey, but this stage was scarce during the spring survey and was not recorded during the autumn survey (Fig. 4). Copepodid abundance was significantly affected by the season of the year and depth strata, exhibiting their highest abundance during the winter survey and in the surface samples (pb.5), while tide and period of the day did not have a significant effect on copepodid abundances (Table 3). Nauplii II abundances were significantly explained by the variables season (highest abundance was observed during winter), period of the day (higher abundance during the day), tide (higher abundance during flood tide), and water density (higher abundance with lower density) (pb.5, Table 3, Fig. 4). Vertical contours of flow from U (east west) current components for transects 2 (Fig. 5A) showed a dominant flow toward the west for almost the entire water column, except for a narrow band at the western end of both transects. This band could be explained by the effect of the dominant flood tide current emanating from the Abtao Channel, which causes a convergence of the flow in the transitional area for both current directions. Across transect 4 this situation was reversed and the dominant flow was in an easterly direction within a narrow vertical band flowing towards the west (Fig. 5C), suggesting that the current from the Abtao channel was divided on the north side of the bay promoting a divergence zone. For the V (north south) component of flow (Fig. 5B and D), the vertical band of positive flow (towards the north) on the left of transect 2 suggested an effect of the flood current from the Abtao Channel, as was observed for transect 4. In this case the portion of the column water affected was wider than that observed in the U component, which suggests the presence of a gyre moving in a clockwise direction in the flow. Vectorial flows at the two selected depths exhibited the tendency of the flow to rotate clockwise in the surface layer, and at a depth of 45 m. However part of the bottom flow (45 m) seemed to leave the bay moving eastwards (Fig. 6A), following the orientation of the submarine valley (Fig. 1). The dominant flow towards the north from the Abtao Channel was more clearly observed in deeper layers than at the surface, as demonstrated by the vertical contours. Vertical density profiles showed slight stratification and the observed density of the water column was slightly higher in the western CTD station (close to Abtao Channel) than in the eastern CTD station (close to Lagartija Island) (Fig. 7) Mode 3: spatial variation Spatial distribution of copepodids and nauplii II larvae was heterogeneous during the 12 h sampling period. Nauplii II ranged from 1 to 12 larvae per 34 m 3 (median=), while copepodids ranged from 1 to 4 larvae per 34 m 3 (median=). Both copepodids and nauplii II were more abundant in the first sample which seemed to be associated with density fields and the flood tide (Fig. 8). During the ebb tide larvae were more abundant towards the mouth of the bay, and in the final sample of the afternoon larvae were rare (Fig. 8). Drifters deployed at 2 m exhibited a larger displacement than surface drifters, although none of them moved more than 2 km inside Fig. 6. A) Velocity vectors of the flow obtained with ADCP profiles at 1-meter and 45-meter depth (cm/s, without decimals). B) Flow direction and displacement of drifters at the surface and at the 2 m.

8 C. Molinet et al. / Aquaculture 312 (211) the bay (Fig. 6B). Surface drifters moved anti-clockwise, while 2- meter drifters drifted clockwise. The latter coincided with the clockwise flow observed during the diurnal studies using ADCP. The GLM model indicated that copepodid larval abundance was not affected significantly by our candidate predictor variables (tide, water density and location) (pn.5). However nauplii II abundances were significantly affected by the location of the larvae (higher abundance inner the bay), density of surface water (higher abundance with higher density), and tide (higher abundance during ebb), which explained 6% of the total deviance (Table 4). 4. Discussion Fig. 7. Average density profiles of the water column from the ends of transect 2. The western station was located in front of the Abtao channel and the eastern station toward the channel mouth. Abundances of copepodids and nauplii II stages of C. rogercresseyi were associated with the estimated monthly abundance of ovigerous females (OF) present on the farmed salmons in Codihue bay. GLM analysis indicated significant effect of farmed salmon biomass, chemical treatment (a higher number of treatments were applied during winter, spring and autumn), and host species, on OF abundances (Table 1), the principal source of infective stage production. The observed continuous OF presence in farmed salmon could supply a continuous of copepodids able to re-infecting salmon. These results support the findings of Tully and Whelan (1993) for L. salmonis produced in the west coast of Ireland, and the reported highest nauplius: copepodid ratios observed nearest to Atlantic salmon farms by Penston et al. (28). By contrast, outside Codihue bay, at a station located upstream from the bay relative to the circulation pattern, presented very low larval abundances (Fig. 2). These observations along with the restricted circulation patterns observed inside the bay suggest that the caligid load on farmed salmon is highly associated with local biomass of salmon. It has been demonstrated that in the Broughton Archipelago of British Columbia that sustained high abundances of infectious larvae should be Fig. 8. Results of the semi-diurnal sample (mode 3) carried out during December 28. The first five panels show copepodids abundance during the 12-hour sampling. The second five panels show nauplii II abundance during the 12-hour sampling. Lines in the panels show density fields. The wave under the nauplii panels shows tide variability during the sample period.

9 7 C. Molinet et al. / Aquaculture 312 (211) Table 4 Results from the sequential deviance analysis for the response variable larval abundance of C. rogercresseyi in mode 3 of sample collection (semidiurnal). Predictive variables were: tide, density water, and location. Degrees of freedom Deviance Df. Deviance P (Nχ 2 ) Null copepodid Tide Water density Location Null nauplii II Tide Water density Location expected near lice infested salmon farms (Krkosek et al., 26), contradicting the suggestions of Brooks (25). Nevertheless, we cannot reject the hypothesis that the arrival of wild fish in the bay could induce the initial infestation by C. rogercresseyi as suggested by a number of authors for other sea lice species that infect farmed and wild salmon populations in other countries (Bron et al., 1993; Costelloe et al., 1998; McKibben and Hay, 24; Revie et al., 22). C. rogercresseyi larval abundance was higher during the summer months which agrees with the observed period of infestation by C. elongatus Nordmann and L. salmonis Krøyer which are increasing during spring and summer (Revie et al., 22; Schram et al., 1998; Wallace, 1998). Also the highest abundance of OF on farmed salmon, would support the higher prevalence of sea lice C. rogercresseyi on farmed salmon during the summer months reported by Bravo et al. (29). These authors suggest that temperature could influence the increase of caligids during summer as reported by González and Carvajal (23) in laboratory studies. Copepodids were more abundant than nauplii II during sample mode 1 and 2, which may be associated with their longer life-span compared to nauplii (González and Carvajal, 23), and/or it could be explained by the tendency of copepodids to accumulate at the surface as observed in this study. This fact maximized its finding probability in the plankton samples, as has been observed by copepodids of L. salmonis (Heuch et al., 1995; Penston et al., 28). Although copepodid abundance was significantly higher at the surface than at 15 m, and 25-meter depth we did not observe any differences in copepodid abundance between day and night neither at surface nor at 15 m, during mode 1 and 2 of sampling (Tables 2 and 3). Unlike that observed in other sea lice copepodids (Genna et al., 25; Heuch et al., 1995), we did not find evidence for vertical migratory behaviour by C. rogercresseyi, which must be a subject for future research. The local circulation pattern observed during this study exhibited two important features. First, local circulation was affected by local topography, particularly during the flood tide when the main flow entering Codihue bay came through the Abtao channel (Fig. 5). This feature may significantly influence caligid loads at farms located inside the bay related to these located in the Abtao Channel, particularly as the flow does not seem to reverse during the ebb tide. Second, the water flow tended to circulate inside the bay at low speed, which would cause larval stages to be retained inside the bay for a longer period, concentrating these larvae as observed for nauplii II at the surface during mode 3 of sampling (Table 4, Fig. 8). This event could increase the probability of salmon re-infestation inside the bay, a situation further exacerbated by the reduction of water movement inside the cage caused by the physical barrier of the net as reported by Costelloe et al. (1998) and Costelloe et al. (1996) on the west coast of Ireland. Circulation patterns in the Chilean inland sea have been described as being highly influenced by local topographic features such as sills, constrictions, channels, embayment and combinations of these which promote retention and dispersion zones (e.g. Aiken, 28; Cáceres et al., 26; Silva et al., 1998; Valle-Levinson and Blanco, 24; Valle- Levinson et al., 21). This information must be included in salmon farming management strategies as a tool for make decisions in the context of the establishment of safe sites which should lead to a minimization of risks and maximization of benefits to all concerned parties (Revie et al., 29). Management strategies implemented around the world have obtained good results (Brooks, 29; Revie et al., 29; The Department of Agriculture, 28), although farmed salmon production in Chile is orders of magnitude higher than others countries. On average management plans for the control of caligids around the world include treatment of farmed salmon when abundances of.5 gravid female lice per fish are exceeded. In our study, we observed an average of 1 gravid female (OF) per fish during the period July 28 to March 29. Currently the distance between farms is insufficient, based on regulations enforced in other countries, but more importantly when considering the particular circulation pattern of the semienclosed area studied. Alternatives strategies for control caligids has been the intensification of chemical treatments and the testing of new products designed to control this pest. However, it has been reported that the effectiveness of these treatments has diminished over time in Chile (Bravo, 23; Bravo et al., 28, 29; Rozas and Asencio, 27). Considering the dynamics of C. rogercresseyi, the biomass of farmed fish and the circulation patterns in Codihue Bay; it seems unlikely that the objective of controlling this pest in the bay is achievable without a drastic reduction in biomass (and the number of farmed salmon). Furthermore it appear imperative that a holistic integrated coastal management approach (including the effect of chemical on the environment), as suggested by Buschmann et al. (29), must be applied in order to evaluate the optimum number of farms operating in a culture cycle inside Codihue Bay. Recently sanitary modifications into Chilean Fisheries Law for salmon aquaculture (resolution N 2117 of August 29) and the zoning of coastal waters (D.O.N 35.64, National Policy of Coastal Border Use) are considered as a first step for promoting better management strategies for aquaculture in Chile. 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