Commercial Scallop Dive Fishery (Port Phillip) · Scallop dive surveys were undertaken within Port...

Commercial Scallop Dive Fishery (Port Phillip) Survey Results for 2014

Not for citation without permission

December 2014 Fisheries Victoria Internal Report Series No. 59

Commercial Scallop Dive Fishery (Port Phillip) Survey Results for 2014

December 2014

Fisheries Victoria Internal Report Series No. 59

Commercial Scallop Dive Fishery (Port Phillip) – Su rvey Results for 2014 ii

Published by the Victorian Government, Department of Environment and Primary Industries, December 2014

© The State of Victoria, Department of Environment and Primary Industries Melbourne December 2014 This publication is copyright. No part may be reproduced by any process except in accordance with the provisions of the Copyright Act 1968.

Authorised by the Victorian Government, 1 Spring Street, Melbourne.

Printed by DEPI Queenscliff, Victoria.

Preferred way to cite this publication: Fisheries Victoria 2014. Commercial Scallop Dive Fishery (Port Phillip) – Survey Results for 2014, Fisheries Victoria Internal Report Series No. 59.

Contact Details: Fisheries Management and Science Branch, Fisheries Victoria PO Box 114, Queenscliff Vic 3225

Copies are available from the website www.depi.vic.gov.au/fishing

For more information contact the DEPI Customer Service Centre 136 186

Disclaimer This publication may be of assistance to you but the State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.

Accessibility If you would like to receive this publication in an accessible format, such as large print or audio, please telephone 136 186, or email [email protected]

Deaf, hearing impaired or speech impaired? Call us via the National Relay Service on 133 677 or visit www.relayservice.com.au

This document is also available in PDF format on the internet at www.depi.vic.gov.au

Commercial Scallop Dive Fishery (Port Phillip) – Su rvey Results for 2014 iii

Contents

Executive Summary 5

Introduction 6

Materials and Methods 8

Field Surveys 8

Laboratory Measurements and Analyses 8

Results 9

Discussion 11

Observed patterns of distribution and abundance 11

Estimates of available biomass 11

Recruitment and future scallop abundance 13

Conclusions 14

Acknowledgements 15

References 16

Commercial Scallop Dive Fishery (Port Phillip) – Su rvey Results for 2014 iv

List of Tables

Table 1. Estimates of available scallop biomass in Port Phillip and the parameters used to make these estimates for the management zones surveyed.

List of Figures

Figure 1. Management zones for the Commercial Scallop Dive Fishery (Port Phillip) showing areas excluded

(recreational fishing only) and Marine Protected Areas (no take zones).

Figure 2. Shell width to weight relationship for scallops sampled over the four management zones surveyed between July

and October 2014.

Figure 3. Distribution of Scallop shell width frequency pooled among sites sampled between July and October within

each of the four management zones.

Figure 4. Area of zone 4 within depth strata showing site locations.

Figure 5. Distribution of survey sites within zone 1 and the northern limit to commercial fishing voluntarily adopted by

industry.

Commercial Scallop Dive Fishery (Port Phillip) – Su rvey Results for 2014 5

Executive Summary

This report was prepared on behalf of the licence owner to provide guidance about potential catches of scallops from defined management zones within Port Phillip Bay.

Scallop dive surveys were undertaken within Port Phillip Bay between July and October 2014 with a focus on areas and depths where the previous dredge fishery was most successful. A total of 73 sites were surveyed of which there were 7 where no scallops could be found.

Key results were:

• The distribution of available scallops (i.e. those above the legal size of 90 mm shell width) showed the highest densities around the north western and south western part of Port Phillip Bay. The eastern part of Port Phillip had relatively low densities of scallops and most of these were under the legal size. Compared with similar surveys undertaken during the 1990s, it appears that the distribution of scallops within Port Phillip has changed.

• The survey data were used to estimate the available biomass of scallops within each of the four management zones surveyed. The total available biomass within these zones is estimated to be 3,629 tonnes. The level of precision for management zone 1 (in the north of Port Phillip Bay) was relatively low and improvements to the design of future surveys will reduce the level of uncertainty in the estimated biomass for this zone.

• There are signs of recruitment in all areas surveyed and this was relatively high in the southern and eastern areas of Port Phillip Bay.

Fishery dependent monitoring will be important over the next few years as the highly variable nature of scallop populations and the response of stocks to commercial fishing pressure is difficult to predict.


Introduction The purpose of this report is to provide an estimate of the available biomass of scallops (Pecten fumatus) determined from dive surveys in Port Phillip conducted between July and October 2014. The report also provides some insight into likely patterns for recruitment over the next couple of years, but this is based on cautious interpretation of length-frequency data. These surveys represent the initial investigation to support management of a dive fishery which has yet to begin harvesting. The extent of the surveys has been dictated by Industry intentions to fish in some zones but not others and therefore they do not represent a complete picture of available biomass over the whole of Port Phillip Bay. The work was undertaken in support of the management objectives for the commercial scallop dive fishery, described in the Commercial Scallop Dive Fishery (Port Phillip Bay) Baseline Management Arrangements (2013) (BMA). These objectives, amongst others, are: Long-term sustainability of the scallop resource (to):

• Ensure that the commercial harvest is kept within limits that are consistent with the long-term sustainability of the fishery; and

• Allow fishing for scallops in a manner that has minimal impact

Existing contemporary knowledge of the scallop resources in Port Phillip is limited and the BMA are therefore relatively conservative. These arrangements describe opportunities for catch limits to be increased if this is supported by data (and analyses) provided by the licence holder. There are 3 stages of potential development for this fishery described in the BMA (table 1 in appendix D) and these stages require progressively more comprehensive (and costly) data collection, analyses and reporting. As this is the first year for this fishery, development is limited to some extent by the absence of a time series of data from which to infer trends. It does, however, provide a baseline for future comparisons and the means by which relative performance of the fishery may be determined. The Commercial Scallop Dive Fishery (the Fishery) is subject to a Total Allowable Commercial Catch (TACC) which is managed using the established Quota Management System (QMS). There are 6 quota management units for Pecten fumatus (one unit per zone – see figure 1). The intention is that this spatial approach will help manage sustainability risks. Catch restrictions by zone will ensure that fishing effort is spread spatially and that information about the resource is collected across the entire embayment. The information provided in this report relates to Pecten fumatus only, as this species was the species of interest for commercial harvest to the licence holder. This report provides spatial details of distribution and abundance and size structure of scallops from zones 1, 3, 4 and 5 within Port Phillip Bay. This information was used to estimate the biomass of scallops above the legal minimum size within each management zone and provides the basis for setting a Total Allowable Commercial Catch.


Figure 1. Management zones for the Commercial Scallop Dive Fishery (Port Phillip) showing areas excluded (recreational fishing only) and Marine Protected Areas (no take zones).


Materials and Methods

Field Surveys This fishery-independent survey uses a stratified random sampling design of six strata (Figure 1) substantially similar to that used to estimate scallop abundance for the commercial scallop dredge fishery from 1982 to 1996, with boundaries based on bathymetry and prior history of scallop abundances in Port Phillip Bay (Coleman 1998). The strata were reconfigured as management zones that coincide as far as practicable with the Port Phillip Bay commercial catch reporting grid, and the number of sampling sites in each management zone was based on historical abundance with more sites allocated to those areas where scallops were previously most abundant. This was intended to provide a more cost-effective and efficient sampling program, provided the zones are treated discretely. Under the BMA it is prescribed that twenty five sites are sampled in each of strata 3,4 and 5 and seven sites are sampled in stratum 1. For each stratum (zone), sites were located within the depth rangemost likely to contain scallops (10 to 20 metres) by using random numbers to generate pairs of grid coordinates. These positions were plotted until the required number of sites had been allocated in each zone. In some cases extra sites were allocated to zones where greater precision was required to support the anticipated focus for harvesting. For example a priority was placed on areas within the Pinnace Channel where tidal flow is high and recent monitoring indicated that water quality was good. In the field, sampling sites were located using a global positioning system (GPS) and at each site two divers swam a different 50x1m belt transect while using underwater breathing apparatus. Scallops down to 25mm shell width (measured in a straight line across the widest point) were collected from each pair of transects and brought on board the research vessel. Scallops less than 90mm were measured to the nearest millimetre and returned to the water as soon as possible. Scallops 90mm and larger were measured and retained. These data provided the number of scallops per 100m2 and size-frequency information for each site.

Laboratory Measurements and Analyses On returning to port, retained scallops were individually weighed and measured. In most cases the entire sample was processed, but at sites where scallops were most abundant, random subsampling was required. This sampling provided information about the shell width-to-weight relationship for whole scallops of harvestable size. These data were pooled for all zones sampled during surveys and used to estimate weights from shell width where weight measurements were unavailable (figure 2). Number of scallops per site was not normally distributed, which is typical for count data of patchily distributed animals, with sufficient bias to make it inappropriate to use the mean value when calculating the total number of scallops per zone. The estimated abundance of scallops per management zone was instead calculated as the median number of scallops per square meter found during surveys within that zone, multiplied by the total area of the zone which falls within the depth range 10–20 metres. The available biomass of scallops (those above 90 mm shell width) was calculated for each zone by either using the weights measured in the laboratory when all of a sample was processed, or estimating weights using the shell width-to-weight relationship where subsampling had occurred. Minor adjustments were made to the shell width-derived estimates of weight based on the differences measured between pooled data and laboratory measured weights for each zone. Shell width data were pooled by zone to provide information about likely patterns and relative strength of recruitment over the next couple of years.


Figure 2. Shell width to weight relationship for scallops sampled over the four management zones surveyed between July and October 2014.

Results

Among the 73 sites surveyed for scallops within Port Phillip between July and October 2014, seven did not have any scallops.

The greatest abundance of scallops available to the fishery (above 90 mm shell width) were at sites located on the north western and south western parts of Port Phillip. In these areas the density of available scallops ranged between 20 and 40 per 100 m2.. There were several sites located in the north eastern part of Port Phillip, particularly near Carrum Bight, where only undersized scallops could be found.

The total biomass of available scallops showed a similar pattern of distribution as scallop abundance, suggesting that the highest biomass of scallops is located within the north western and south western areas of Port Phillip. At most of the sites the measured total weights of samples were between 0.5kg and 4.0kg per 100 m2.

Table 1. Estimates of available scallop biomass in Po rt Phillip and the parameters used to make these es timates

for the management zones surveyed.


The available biomass was estimated for each management zone in tonnes wet weight (table 1). There was no surveying in zones 2 and 6. Investigation of the data collected from the four management zones where surveys were undertaken showed that:

• Zone 3 has the largest area within the chosed depth strata and zone 4 has the smallest area,

• Zones 4 and 5 showed signs of a more normal distribution of weights around the mean than zones 1 and 3, however due to this disparity the median was used for consistency in the analysis,

• Greater levels of uncertainty were seen in estimates of available biomass for zones 1 and 4 than for 3 and 5

• Total available biomass for zones surveyed (between depth strata 10-20 m) is 3,629 tonnes.

The shell width-weight relationship is based on scallop data collected from throughout Port Phillip between July and October 2014 (r 2 = 0.80). For sites where less than 100% of scallops were weighed (i.e. subsampling of individuals to be weighed occurred), weights were estimated from shell width using this relationship and the results compared with an average of the weights measured for the relevant zone. Weight adjustment factors were then determined for each zone and applied to the sites where weights were estimated. This effectively corrected the bay-wide relationship for each zone. The results show that the proportion of scallops over 90 mm shell width was greatest in zone 1 (74%) and least in zone 4 (22%).

Figure 3. Distribution of Scallop shell width frequency pooled among sites sampled between July and October within each of the four management zones.

Scallops collected during surveys ranged in size from 20 mm to 130 mm shell width, althought the smallest sizes were likely under-reported due to their higher difficulty of detection (Figure 3). The size distribution for each zone shows a consistent bimodal distribution with a peak for smaller scallops at between 45 to 55 mm shell width, and another between 90 to 100 mm shell width. These are likely to be different year classes, but how many year classes are included in each peak is unknown. The peak of smaller scallops indicates that there has been some recruitment to all zones and provides an indication of the relative strength of the fishery – assuming constant survival among zones.

In view of this assumption, caution must be used when interpreting the results, however the data indicate stronger recruitment in zones 4 and 3 compared to zones 1 and 5.

0

10

20

30

40

50

60

5 15 25 35 45 55 65 75 85 95 105 115 125 135

Fre

qu

en

cy

Shell width (mm)

Zone 1

0

10

20

30

40

50

60

70

80

5 15 25 35 45 55 65 75 85 95 105 115 125 135

Fre

qu

en

cy

Shell width (mm)

Zone 3

0

10

20

30

40

50

60

70

80

5 15 25 35 45 55 65 75 85 95 105 115 125 135

Fre

qu

en

cy

Shell width (mm)

Zone 4

0

50

100

150

200

250

300

5 15 25 35 45 55 65 75 85 95 105 115 125 135

Fre

qu

en

cy

Shell width (mm)

Zone 5


Discussion

Observed patterns of distribution and abundance Dive survey results describe the distribution and abundance of available scallops prior to commercial fishing. This provides information useful at the operational level, but it is only a representation of where scallops were found during the survey period (July-October). Population dynamics will be strongly inlfuenced by a range of envrionmental variables that cause recruitment, survival and growth to vary to the extent that the available stock will be prone to fluctuate from year to year. Patterns in available biomass may change as a result of differential mortality within and between zones as time goes by. The biomass of scallops found over the survey period may also change as a result of food availability and, perhaps more significantly, gonad condition which is expected to decline during summer months (Coleman 1998).

These patterns will be even more variable when commercial fishing commences, and it is due to this variability that Baseline Management Arrangements require fishery-independent surveys to be conducted annually prior to each fishing season.

This survey did not cover depths greater than 20 m and less than 10 m. These limits were based primarily on historical information suggesting that scallops are seldom found outside the 10-20m depth stratum, but also partly because of the increased risks associated with hyperbaric exposure for divers working at greater depths. Similarly, no sampling was undertaken in zone 2, zone 6, or the northern part of zone 1, either due to industry concerns about diver safety at greater depths or concerns regarding water quality. Areas closed to commercial harvest for any reason (including to provide areas exclusively for recreational fishing) provide a refuge from commercial fishing that will help maintain the spawning biomass. If commercial harvesting is restricted to areas as described above, then 24% of the total area of Port Phillip will be subject to commercial scallop fishing.

Shipping channels were deliberately excluded from survey areas because they represent a significant hazard to diving operations.

Estimates of available biomass A minimum number of sites per zone is prescribed under the BMA based on levels of precision reported during previous (similar) surveys of scallops in Port Phillip (Coleman 1998). The minimum number of sites sampled per zone was exceeded except for zone 4 where only 8 sites were sampled. This undersampling was due to the limited area open to fishing within the 10–20 m depth strata (Figure 4) and the need to avoid shipping channels. During previous surveys this zone was usually not sampled given the prevalence of sandbanks and the fact that it was seldom fished by the dredge fishery (Coleman 1998). The observed precision (CV=36%) suggests that there is greater uncertainty about the estimated biomass of 64 tonnes within the zone when compared with zones 3 and 5.

Similarly, within zone 1 the number of sampling sites was relatively low (n=8) compared to zones 3 and 5, and the precision (CV=42%) suggests that the 1366 tonne estimate of available biomass should be used with caution. The allocation of sites within this zone appears to be spatially uneven (Figure 5) and further sampling within this zone would improve the precision of the biomass estimate.

The level of precision was highest for zones 3 and 5, which collectively account for an estimated biomas of 2199 tonnes, and conformed to the level (CV<25%) specified in the BMA. Until 1998 there had been a consistent trend in both the abundance of scallops found by surveys and the commercial catches recorded from the most productive areas of the fishery - zones 3 and 5 (Coleman 1998). This demonstrated the value of dive surveys for predicting the performance of the fishery within cerain management zones. The value of dive surveys for predicting performance of the new dive fishery is yet to be determined. The distribution of scallops appears to have changed since the last surveys in the 1990s but it will be interesting to see if this is reflected in the commercial harvest.

Given the substantial areas which were not sampled, the current design cannot provide an estimate of the total biomass of scallop within Port Phillip. It should also be expected that some scallops could be found outside the depth strata we sampled (10-20 meters) and therefore our estimates of biomass within each zone are also conservative.


Figure 4. Area of zone 4 within depth strata showing site locations.


Recruitment and future scallop abundance It is important that high quality fishery dependent data are collected from the outset of fishing and part of this includes mandatory reporting. It is likely that the biomass of scallops will reduce as the fishery initially develops, but this may not be detected if prevailing environmental conditions are highly favourable. This is because scallops have a relatively strong compensatory mechanism for recovery in relatively short time frames when the conditions are favourable.

Within the first few years of fishing it is also likely that fishery dependent data will fluctuate due to spatially shifting patterns of effort. Under some scenarios this can lead to hyperstability when new grounds are continually fished and there is no perceived impact of harvesting on the stock. This means that the fishery performance indicators based on catch, effort and catch rate, must be rigorously monitored at the initial stages of fishery development and must be acquired consistently from year to year to establish a time series suitable for medium to longer-term montoring..

Legal minimum size (90 mm shell width) is set to allow for two spawning seasons on average prior to scallops recruiting to the fishery. This provides protection of the spawning biomass even under scenarios of high exploitation rates and some opportunity for replenishment of the resource. It is relevant that there was no legal minimum size for scallops while the dredge fishery operated , however on-board monitoring of meat weights served a similar purpose and was used to close the fishery when the proportion of small scallops exceeded a pre-determined threshold.

Recruitment variability in the Port Phillip dredege fishery is reflected in the annual yield which has ranged from 2000 tonnes meat weight to 0. Even in less extreme circumstances, the yield would vary almost 20-fold between years (Coleman 1998). The usefulness of dive surveys was supported by the positive correlation between the abundance of scallops estimated from surveys and the yield from the fishery, however no correlation could be found between the abundance of newly settled scallops (spat) and population abundance one and two years later. At post settlement stages it may be expected that scallops are vulnerable to a wide range on environmental effects (including predation) and broad scale mortality could most impact on populations.

The current study indicates that the observed peak of scallops between 45–55 mm shell width will be reflected in higher levels of recruitment to the stock during 2015 in zones 4 and 3 compared to zones 1 and 5. The most likely time to detect this recruitment will be April-May and fishery dependent monitoring can be adapted for this purpose

Figure 5. Distribution of survey sites within zone 1 and the northern limit to commercial fishing voluntarily adopted by industry.


Conclusions

The Commercial Scallop Dive Fishery (Port Phillip) will operate during the next year over less than twenty five percent of the total area of Port Phillip Bay and the industry has indicated that this is not likely to change in future. Apart from exclusion zones set aside for recreational scallop harvesting, these areas are where commercial scallop fishing has previously been most successful. The recent scallop dive survey found the greatest abundance of available biomass in the north-western and south-western zones, however quite large areas of Port Phillip were not covered by the surveys. In addition, survey sites were restricted to depths between 10 and 20 meters. These constraints mean that the results cannot be used to estimate the total number (or biomass) of available scallops within Port Phillip and the estimates of available biomass within zones are also likely to be conservative.

The report discusses the extent and likely causes of the different levels of precision and ways that this may be improved for future surveys. It is estimated that 3,629 tonnes of scallops are available to be fished within the four zones surveyed and this information will be used as the basis for setting Total Allowable Commercial Catches for each of these zones. There has been recruitment within all zones surveyed but this recruitment was variable between zones. Because of this variability and the unknown response of the fishery to commercial havesting, it is recommended that careful monitoring of both fishery dependent and pre-season survey data is continued over the next few years.


Acknowledgements

DEPI Fisheries Management and Science staff and specialist scientific diving contractors contributed collectively to different aspects of the work presented in this report. This report is a simple summary underpinned by substantial effort expended in the fileld, laboratory, during consultative meetings, and throughout the report drafting process.

The study was funded entirely by Port Phillip Scallops Pty Ltd in accordance with DEPI cost-recovery policy.


References

Coleman, N. (1998) Counting scallops and managing the fishery in Port Phillip Bay, south-east Australia. Fisheries Research 38, 145-157. Fisheries Victoria (2013) Commercial Scallop Dive Fishery (Port Phillip Bay) Baseline Management Arrangements (2013). http://tinyurl.com/PPB-BMA-2013 Accessed on 20 Nov 2014.

Commercial Scallop Dive Fishery (Port Phillip) · Scallop dive surveys were undertaken within Port...

Documents

Transcript of Commercial Scallop Dive Fishery (Port Phillip) · Scallop dive surveys were undertaken within Port...