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Stocking density and rearing environment affect external condition, gonad quantity and gonad grade in onshore sea urchin roe enhancement aquaculture
Aquaculture 515 (2020) 734591
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Stocking density and rearing environment affect external condition, gonad quantity and gonad grade in onshore sea urchin roe enhancement aquaculture
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Fletcher Warren-Myersa,b,∗, Stephen E. Swearerb, Kathy Overtona, Tim Dempstera a b
Sustainable Aquaculture Laboratory – Temperate and Tropical (SALTT), School of BioSciences, University of Melbourne, Victoria, 3010, Australia National Centre for Coasts and Climate (NCCC) and School of BioSciences, University of Melbourne, Victoria, 3010, Australia
ARTICLE INFO
ABSTRACT
Keywords: Barrens Culture conditions Heliocidaris erythrogramma Intraspecific competition Temperate urchin
Finding the ideal density to optimise growth, health and welfare of aquaculture species reared in cage or tank environments allows farmers to produce the best quality product per unit area. Appropriate stocking densities are well known for most major aquaculture species, but limited information exists for the developing sea urchin aquaculture industry. The temperate purple sea urchin Heliocidaris erythrogramma is a prime candidate for aquaculture, yet how stocking density and/or the rearing environment influence urchin health and roe production remain unknown. Here, we tested whether stocking density (low vs. high) and rearing environment (individual vs. group) influences urchin external condition, gonad index (GI) and roe grade in H. erythrogramma after 12-weeks of roe enhancement during autumn-winter (April–June) and winter-spring (July–September) periods. Across rearing environments, high density reduced the proportion of urchins with healthy external condition by 26–30% during autumn-winter and 7–20% in winter-spring compared to low density. During the autumn-winter period, while higher density resulted in lower GIs in the group reared treatments, higher density did not affect the GI of individually reared urchins. Rearing environment affected gonad grade in the autumnwinter period, with a higher proportion of quality (B-grade) roe produced in individually reared treatments compared to group reared treatments. Our results highlight the importance of stocking density and rearing environment for onshore urchin roe enhancement. Maximising production will require a trade-off between investing in greater infrastructure cost to individually rear urchins at high density to ensure higher GIs versus investing in less expensive infrastructure, but group rearing urchins at lower densities to ensure high GIs after enhancement.
1. Introduction Stocking density is a critical production parameter for aquaculture species, as density can influence an individual's growth rate, health and welfare (Stien et al., 2013). Stocking density affects both highly mobile cultured species (e.g. Atlantic salmon Salmo salar; Turnbull et al., 2005; rainbow trout Oncorhynchus mykiss; Ellis et al., 2002; Nile tilapia Oreochromis niloticus L.; El-Sayed, 2002; gilthead sea-bream, Sparus aurata L.; Canario et al., 1998) and less mobile benthic species, (e.g. blacklip abalone Haliotis rubra; Huchette et al., 2003; prawns Macrobrachiurn rosenbergii; Tidwell et al., 1999; sea cucumbers Apostichous japonicus; Li and Li, 2010; Xia et al., 2017). Density can affect growth either directly through competition for space or food, or indirectly through the buildup of excretory products (Huchette et al., 2003). Some species thrive
when held in high density conditions (e.g. Arctic charr Salvelinus alpinus; Jørgensen et al., 1993) while other species experience reduced growth and/or have higher mortality rates (e.g. red tilapia Oreochromis sp.; Suresh and Lin, 1992). For species new to culture, such as sea urchins undergoing roe enhancement, relatively little is known regarding optimal stocking practices. Sea urchins can occur in densities as high as 80–120 urchins m−2 (Grisolia et al., 2012; Kriegisch et al., 2016) on rocky reefs when they become overabundant and form urchin barrens. When collected from barrens, their gonads are often unmarketable due to very low gonad indices (GIs) (Pert et al., 2018). Low GIs in urchins from barrens are likely due to limited food availability, as urchin densities ∼8 m−2 are enough to limit algal food availability on barrens due to overgrazing (Kriegisch et al., 2016). Roe quality in the temperate urchin Evechinus
∗ Corresponding author. Sustainable Aquaculture Laboratory – Temperate and Tropical (SALTT), School of BioSciences, University of Melbourne, Victoria, 3010, Australia. E-mail address: [email protected] (F. Warren-Myers).
https://doi.org/10.1016/j.aquaculture.2019.734591 Received 28 June 2019; Received in revised form 9 October 2019; Accepted 9 October 2019 Available online 15 October 2019 0044-8486/ © 2019 Elsevier B.V. All rights reserved.
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cool box−1) while in transit to the land-based aquaculture facility in Queenscliff, Victoria. On arrival at the facility, urchins were haphazardly selected from cool boxes and placed into 9 L trays (Internal width 0.26 m, length 0.35 m, height 0.10 m; total surface area 0.213 m2). Each tray sat immersed in the top half of a full 18 L holding tank with either 5 or 10 urchins tray−1 (low or high density), individually separated from each other (individually reared) or allowed to interact with each other (group reared) in the tray (Fig. 1). Each tank set up (18 L holding tank + 9 L tray) received ambient seawater via a 19 mm tap at a continuous flow rate of 0.5–1 L min−1. Each 9 L tray was fitted with a grilled mesh floor (4 mm2 hole size) to allow faecal matter to pass through to a sediment trap at the bottom of each 18 L holding tank. Ambient seawater temperature in the onshore facility progressively dropped from 18.1 to 13.6 °C during the autumn-winter period and remained between 12.8 and 13.5 °C for the winter-spring period. Urchin test diameters ranged from 47.1 ± 0.6 to 48.7 ± 0.6 (mean ± SE) across treatments for the autumn-winter enhancement and 43.2 ± 0.2 to 44.0 ± 0.3 (mean ± SE) across treatments for the winter-spring enhancement (Table 1).
chloroticus appears to depend more on the availability of feed rather than stocking density (Andrew, 1986; James, 2006). However, this does not hold true for all species as in the cool water urchin Strongylocentrotus droebachiensis, increased stocking density did not alter feed intake but negatively affected gonad growth (Siikavuopio et al., 2007). The negative effects of increasing stocking density appear more pronounced for warmer water species of urchins. Among tropical species, stocking densities of 40–180 urchins m−2 (Lytechinus variegatus, Richardson et al., 2011) and 43 to 129 urchins m−2 (Tripneustes gratilla, Mos et al., 2015) have been tested in laboratory experiments. For both species, high stocking density produced negative outcomes. L. variegatus stocked at 180 urchins m−2 resulted in approximately 18% cannibalism, whilst there was no effect at 40 urchins m−2 (Richardson et al., 2011). For T. gratilla, growth rates and relative spine lengths all decreased in high density culture (Mos et al., 2015). The purple sea urchin (Heliocidaris erythrogramma) is an endemic species that inhabits temperate coastal waters in southern Australia (Evans and Marshall, 2005; Pert et al., 2018). In Port Phillip Bay, Victoria, there is an estimated 4300 tons of purple sea urchins, the majority of which reside in urchin barrens at densities ranging from 20 to 120 urchins m−2 (Worthington and Blount, 2003; Johnson et al., 2015; Kriegisch et al., 2016). Local commercial fisheries rarely harvest sea urchins from the barrens due to poor quality roe, which makes them unfit for sale (Sanderson et al., 1996; Pert et al., 2018). However, recent studies have demonstrated that the roe quality of purple sea urchins in barrens can be improved via roe enhancement aquaculture when using high quality feed and optimal environmental conditions (Pert et al., 2018). Previous research on enhancing the roe of purple sea urchins harvested from barrens has primarily focused on feed quality (Pert et al., 2018) and harvesting methods (Warren-Myers et al., 2019a) and used a single stocking density of 10–12 adult urchins per 18–20 L tank. However, optimal stocking density remains unknown as does whether culturing urchins individually or in groups, most benefits roe enhancement. Here, we used two rearing densities, 5 adult urchins per 9 L tank and 10 adult urchins 18 L tank, and cultured urchins individually or in groups, to test if external condition, gonad indices, or gonad grade of urchins collected from barrens habitats were affected. A better understanding of the appropriate rearing environment and optimal stocking density will help fast track the commercial development of roe enhancement aquaculture for H. erythrogramma harvested from urchin barrens.
2.3. Gonad enhancement Urchins were roe enhanced with a pelleted feed that contained optimal levels of lipid and protein for gonad enhancing Heliocidaris erythrogramma (Warren-Myers et al., 2019b) supplied by the Nutrition and Seafood Laboratory (NuSea.Lab), Warrnambool, Victoria, Australia. Feeding (0.6 g urchin−1) occurred three times week−1 for 12 weeks. Faecal matter was siphoned from the settlement area of the 18 L tanks weekly. At 12 weeks post-collection, gonads were dissected, and a gonad index (GI) was estimated using the formula:
% Gonad index =
Urchin gonad wet weight × 100 Urchin total wet weight
2.4. External condition Urchin external condition was assessed by visual assessment and urchins were classified into three classes at the end of the 12-week roe enhancement period as per Warren-Myers et al. (2019a). Classifications were healthy (no spine loss), average (<25% spine loss), or poor (>25% spine loss or with scarring or blemish marks).
2. Methods
2.5. Gonad grade
2.1. Tank environment
Gonads were graded to assess the commercial quality of H. erythrogramma roe based on colour, texture and firmness by following the method and grading table outlined in Pert et al. (2018). Gonad colour was separated into 3 colour categories: (1) bright yellow and orange; (2) pale yellow and orange or dark yellow and orange; and (3) black, brown or grey. Gonads were then separated into 3 texture categories: (1) fine (<1 mm granulation); (2) medium (between 1 and 2 mm granulation); and (3) coarse (>2 mm granulation). Finally, gonads were deemed either (1) firm or (2) not firm based on whether they did or did not hold together during dissection. The grading table (Table 2) was then used to categorise gonads into A, B, C or D grade, were A grade is considered premium quality, B grade high quality, C grade mediocre quality and D grade unacceptable commercial quality.
Urchin density (low or high) and rearing environment (individual or group) (Fig. 1) were manipulated during two separate roe enhancement periods (autumn-winter from April–June and winter-spring from July–September), to assess their effects on external health, gonad production, and gonad grade. The two roe enhancement periods were chosen to align with the recovery, growing and pre-mature stages of the reproduction cycle of H. erythrogramma which typically extends from March to October each year (Pert et al., 2018). 2.2. Collection and tank setup Divers collected urchins for the autumn-winter period on the 4th April and for the winter-spring period on the 10th July 2018. Urchins were collected using a hook and catch bag method (Warren-Myers et al., 2019a) from a rocky urchin barren in the northern part of Port Philip Bay (PPB) in Victoria (PPB; 37.87° S, 144.85° E), approximately 0.5 km offshore. PPB sea surface temperature at the time of each collection date was approximately 18 °C and 13 °C (respectively). Urchins were held in 33 L cool boxes, aerated with pure O2 (0.5–1 L min−1 of O2
2.6. Statistical analysis Mean proportion of healthy urchins, mean gonad indices (%GI) and mean roe grade proportions for each roe enhancement period were analysed using a series of two-way ANOVAs (factor 1, rearing environment = individual vs. group reared; factor 2, density = low vs. high). Untransformed data were used for analysis as raw data met 2
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Fig. 1. The four treatment types. For group reared treatments, low density = 5 urchins. tray−1 and high density = 10 urchins. tray−1. For individually reared treatments, at low density 5 of 6 cells contained urchins and for high density, 10 of 12 cells contained urchins. There were 6 replicate tanks per treatment. Table 1 Rearing environment, stocking density and external condition at harvest for roe enhancement during the autumn-winter and winter-spring. Urchin survival to harvest was 100% for all treatments. Treatment
Replicate
Rearing (I = Individual, G = Group) Density (L = Low, H = High) Autum-Winter IL IH GL GH Winter-Spring IL IH GL GH
Count
Diameters
Stocking density a
Final external conditon
Urchins
Test
Test + spine
Urchins
Urchins
Percentage cover
(n)
(per tray)
(mm ± SE)
(mm ± SE)
m−2
L−1
(% ± SE)
6 6 6 6
5 10 5 10
47.6 ± 0.8 47.1 ± 0.6 48.7 ± 0.6 47.3 ± 0.2
87.2 ± 1.0 86.4 ± 0.9 88.6 ± 1.2 86.7 ± 0.2
23.5 47 23.5 47
0.6 1.1 0.6 1.1
14.0 ± 0.3 27.5 ± 0.5 14.5 ± 0.4 27.8 ± 0.2
100 ± 0 82 ± 18 100 ± 0 70 ± 8
0 16 ± 13 0 22 ± 5
0 2±2 0 8±5
6 6 6 6
5 10 5 10
44.0 ± 0.3 43.5 ± 0.4 43.8 ± 0.3 43.2 ± 0.2
82.9 ± 0.6 81.8 ± 0.4 83.1 ± 0.9 81.0 ± 0.5
23.5 47 23.5 47
0.6 1.1 0.6 1.1
12.7 ± 0.5 24.7 ± 0.6 12.7 ± 0.7 24.2 ± 0.7
97 ± 4 93 ± 4 97 ± 4 80 ± 8
3±3 7±3 3±3 10 ± 4
0 0 0 10 ± 4
Healthy
Average
Poor
% ± SE)
a
Test + spine diameters are estimated from test diameters using the linear model y = 1.3504x + 22.847, R2 = 0.782 which was fitted to test and test + spine diameter data measured from a subsample of 30 urchins.
ANOVA assumptions of homogeneity of variances and normality. All values are reported as the mean ± 1 Standard Error (SE). Urchin density is reported (Table 1.) as number urchins per m2 of internal surface area of tank, number urchins per L, and estimated percentage cover of internal tank surface area using maximum urchin diameter (urchin test + spines) (James et al., 2017) to allow for comparisons between this study and other research.
healthy external condition range from 70 to 100% during the autumnwinter period and 80–97% during the winter-spring roe enhancement period (Table 1). During the autumn-winter, there was no interaction between density and rearing environment (F1, 23 = 0.1, p = 0.5) and no effect of rearing environment (F1, 23 = 0.1, p = 0.5), but there was an effect of density (F1, 23 = 9.8, p = 0.005). At the end of the enhancement, both low density treatments had 100% of urchins in healthy condition, whilst urchins cultured at high density averaged 70 ± 8% for group reared urchins and 82 ± 18% for individually reared urchins (Table 1). The same pattern was observed during the winter-spring enhancement period, with no interaction between density and rearing environment (F1, 23 = 2.6, p = 0.1) or an effect of rearing environment (F1, 23 = 2.6, p = 0.1), but an effect of density (F1, 23 = 6.1, p = 0.023).
3. Results 3.1. External condition Across treatments, the proportion of urchins classed as having 3
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Table 2 The grading system used to assess the commercial quality of H. erythrogramma roe based on colour, texture and firmness. A grade is premium quality, B grade is high quality, C grade is mediocre quality, while D grade is of unacceptable commercial quality. ‘F’ is a firm gonad, while ‘NF’ is not firm (Pert et al., 2018).
Table 3 ANOVA summary of effect of rearing environment and density on gonad grade distributions during autumn-winter (April–June) and winter-spring (July–September) roe enhancement periods. *indicates significant effect at p < 0.05.
Colour
Texture
Firmness
Grade
Autumn - winter
Winter - spring
Bright orange or yellow
Fine
F NF F NF F NF F NF F NF F NF F NF F NF F NF
A B A B B B B C B C C C D D D D D D
F1,23
P
F1,23
p
0.756 1.345 4.118
0.395 0.260 0.056
0.179 0.738 0.000
0.676 0.400 0.990
3.034 0.150 6.330
0.097 0.703 0.021*
0.190 4.198 0.001
0.668 0.054 0.980
0.016 0.016 1.958
0.900 0.900 0.177
0.054 0.054 0.054
0.819 0.819 0.819
1.984 2.770 0.160
0.174 0.112 0.899
0.044 2.792 0.044
0.837 0.110 0.837
Medium Coarse Pale/dark orange or yellow
Fine Medium Coarse
Black, brown, grey
Fine Medium Coarse
A grade Density*Rearing environment Density Rearing environment B grade Density*Rearing environment Density Rearing environment C grade Density*Rearing environment Density Rearing environment D grade Density*Rearing environment Density Rearing environment
Both low density treatments averaged 97 ± 4% of urchins in healthy condition after 12 weeks of roe enhancement, whilst urchins cultured at high density averaged 80 ± 8% for group reared urchins and 93 ± 4% for individually reared urchins (Table 1).
(Table 3, Fig. 3). Individually reared treatments produced an average of 68 ± 5% B grade roe, whereas group reared treatments produced 46 ± 8% B grade roe. There were no other significant effects of rearing environment or density on gonad grade distribution (Table 3).
3.2. Gonad indices
4. Discussion
After roe enhancement, GIs ranged from 13 to 17% during the autumn-winter and 14–16% during the winter-spring periods across treatments (Fig. 2). During autumn-winter, there was an interaction between density and rearing environment (F1, 23 = 8, p = 0.013) as rearing environment influenced the effects of density on final GIs. Lowdensity group reared urchins produced higher GIs than high-density group reared urchins, but there was no difference in GI between low and high densities in the individually reared treatments and the lowdensity group reared treatment (Fig. 2). During the winter-spring enhancement period, there was no interaction between density and rearing environment (F1, 23 = 1.0, p = 0.3) and no effect of density (F1, 23 = 4.0, p = 0.1) or rearing environment (F1, 23 = 0.6, p = 0.4, Fig. 2).
Optimal culturing conditions for Heliocidaris erythrogramma were dependent on both stocking density and whether urchins were reared individually or in groups. Here we demonstrated that higher stocking densities negatively affect the external condition of urchins regardless of how they were reared. Higher density also reduced final GIs after roe enhancement, but this effect can be mitigated if urchins are isolated from one another in the rearing environment rather than reared in groups. 4.1. External condition The external condition of H. erythogramma at harvest was best under low stocking densities for both roe enhancement periods. The external appearance of an urchin is important for marketability if they are sold as a live export product, as roe quality is directly linked with external appearance in certain species of urchin (Mos and Dworjanyn, 2019). The occurrence of external injuries is also a known indicator of poor gonad growth (Siikavuopio et al., 2007). The greater occurrence of external injuries (spine loss or damage) with higher stocking density is most likely due to intraspecific competition for space which may cause increased animal stress. For example, the urchin Lytechinus variegatus attacks and cannibalises other urchins when held in high density (180 urchins m−2) (Richardson et al., 2011). Decreased spine lengths can also occur in high density culture (Tripnuestes gratilla: Mos et al., 2015). Furthermore, Siikavuopio et al. (2007) found that mortality and the occurrence of injuries dramatically increased when stocking densities exceeded 86 urchins m−2 for the adult (>70 g) green sea urchins (Strongylocentrotu droebachiensis). Rearing environment had no overall effect on external condition. While isolating urchins stopped any direct effects of intraspecific competition resulting in injuries or spine loss, any benefits of improving external health by rearing urchins individually may have been offset by the confined space restricting movement, hence causing spine damage or loss. Siikavuopio et al. (2007) used individually compartmentalised units (12 × 12 × 18 cm) for urchins in a density trial on S.
3.3. Gonad grade Rearing environment influenced the proportion of high-quality (Bgrade) gonads during the autumn-winter roe enhancement period
Fig. 2. Gonad indices (GI%) measured in the purple sea urchin Heliocidaris erythrogramma after 12 weeks of roe enhancement during the autumn-winter (April–June) and winter-spring (July–September) periods. Low density = 5 urchins. tray−1, high density = 10 urchins. tray−1. Error bars ±1 Standard Error. 4
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Fig. 3. Gonad grade distribution measured in the purple sea urchin Heliocidaris erythrogramma after 12 weeks of roe enhancement during the autumn-winter (April–June) and winter-spring (July–September) periods. Low density = 5 urchins. tray−1, high density = 10 urchins. tray−1. Group or individually refers to rearing environment in tray. N = 6 tanks per treatment. Gonads were graded into A, B, C, or D grade following Pert et al. (2018) methods. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
droebachiensis and reported no external injuries when urchins were held individually. However, their individual compartments (0.0026 m3 for each urchin) were approximately 1.7–3.4 times larger in total volume than the individual compartments in our individual rearing treatments (Low density - 0.0015 m3, High density - 0.00076 m3, for each urchin, respectively) meaning any effect on external condition due to restricted movement in a confined space may not have arisen in their study. Determining the optimal minimum compartment size to individually house urchins to ensure healthy external condition will require further investigation as compartment size may also be a function of urchin size.
and all urchins were sourced from the one collection site in PPB. Hence, why roe grade was lower in individually reared treatments is unclear. 4.4. Autumn-winter vs. winter-spring enhancement Final GIs differed across treatments during the autumn-winter enhancement, but not during winter-spring enhancement. This may have been due to differences in seasonal temperature profiles (James et al., 2007) during each period, more so than the difference in average size of urchins used (Pearce et al., 2004) for each period. Ambient seawater temperature in the autumn-winter period dropped from 18.1 to 13.6 °C but was cooler and more stable between 12.8 and 13.5 °C for the winterspring period. Average urchin test size was ∼10% larger during the autumn-winter compared to the winter spring period (Table 1). Urchins during the autumn-winter period may have been more active due to experiencing higher water temperatures (Percy, 1973), and coupled with a slightly larger average test size, this may have led to greater competitive interactions. This may explain the differences observed in GIs amongst treatments during the autumn-winter enhancement period (autumn-spring GIs 13.2–17.4%; winter-spring GIs 14.3–16.2%) but not the winter-spring period.
4.2. Gonad indices Gonad indices (GIs) exceeded the recommended marketable minimum of 10–12.5% (Lawrence et al., 2011; Pert et al., 2018) across all treatments and ranged from 13.2 to 17.4% during autumn-winter and 14.3–16.2% in winter-spring. Final GIs differed between the high and low density for group reared urchins, but not individually reared urchins, only during the autumn-winter enhancement period. An interaction between rearing environment and urchin density during the autumn-winter enhancement highlighted that rearing urchins individually during roe enhancement reduced the difference in final GI between the low density (5 per tray) and high density (10 per tray) treatments. Social interaction can increase in sea urchins with increasing density (Qi et al., 2016), hence eliminating interactions and/or competition between urchins through rearing environment may have allowed a greater amount of metabolic energy to be allocated to gonad growth. The results indicate that the decision of whether to cultivate urchins individually or in groups will be a trade-off between desired GI per urchin and number of urchins per unit area cultured. Here, while the low-density group reared treatment produced the overall highest GIs for both enhancement periods (Fig. 2), in terms of production per unit area, the high density individually reared urchins would produce approximately 1.9 times more gonad biomass than low-density group reared urchins.
4.5. Tank design Tank design, water exchange, and the way urchins a housed are important factors to consider when designing an urchin aquaculture system (e.g. Motnikar et al., 1998; Daggett et al., 2006; Siikavuopio et al., 2007; James, 2013; James and Siikavuopio, 2015). Here, urchins were housed in shallow 10 cm deep trays, that sat inside holding tanks (total depth ∼ 20 cm) with a high-water exchange rate (2–4 times per hour). No mortalities occurred during enhancement, however scaling up to a commercial level may have potential issues, as tank size (e.g. large race ways vs. small race ways Daggett et al., 2006) or lower water exchange rates may result in higher mortalities (Motnikar et al., 1998) or poorer growth rates (Mos et al., 2016). Caging urchins individually in shallow tanks limits intraspecific competition, which allows for increased urchin density while still maintaining marketable gonad quantities. However, extra physical structures in tanks used to keep urchins separated could cause issues with cage cleaning, or the potential for areas of larger shallow tanks to have poorer water exchange. Implementing the use of bucket tippers, in shallow race way tanks could be a way to increase water movement and assist with cleaning individual cage structures (e.g. James and Siikavuopio, 2015). This would allow for urchins to be housed individually in cages suspended in shallow raceways, while still maintaining suitable water circulation.
4.3. Gonad grade A greater proportion of B-grade roe was produced in the individually reared treatments (58–76%) compared to the group reared treatments (40–52%) during the autumn-winter enhancement period. More B grade roe in the individually reared treatments appeared to offset by a combination of less A or C grade roe during the autumnwinter, but this pattern did not hold for the winter-spring enhancement period (Fig. 3). In H. erythrogramma, feed type influences roe grade (Senaratna et al., 2005; Pert et al., 2018) as does the collection site (Pert et al., 2018). However, we used only one feed type for all treatments 5
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5. Conclusion
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Richardson, C.M., Lawrence, J.M., Watts, S.A., 2011. Factors leading to cannibalism in Lytechinus variegatus (Echinodermata: Echinoidia) held in intensive culture. J. Exp. Mar. Biol. Ecol. 399, 68–75. Sanderson, J.C., Le Rossignol, M., James, W., 1996. A Pilot Program to Maximise Tasmania's Sea Urchin (Heliocidaris Erythrogramma) Resource. Fisheries Research & Development Corporation, Canberra. https://eprints.utas.edu.au/11926/1/ JCSUrchFRDCChpt0_1_2_3.pdf, Accessed date: 10 April 2019. Senaratna, M., Evans, L.H., Southam, L., Tsvetnenko, E., 2005. Effect of different feed formulations on feed efficiency, gonad yield and gonad quality in the purple sea urchin Heliocidaris erythrogramma. Aquacult. Nutr. 11, 199–207. Siikavuopio, S.I., Dale, T., Mortensen, A., 2007. The effects of stocking density on gonad growth, survival and feed intake of adult green sea urchin (Strongylocentrotus droebachiensis). Aquaculture 262, 78–85. Stien, L.H., Bracke, M.B., Folkedal, O., Nilsson, J., Oppedal, F., Torgersen, T., Kristiansen, T.S., 2013. Salmon Welfare Index Model (SWIM 1.0): a semantic model for overall welfare assessment of caged Atlantic salmon: review of the selected welfare indicators and model presentation. Rev. Aquac. 5, 33–57. Suresh, A.V., Lin, C.K., 1992. Effect of stocking density on water quality and production of red tilapia in a recirculated water system. Aquacult. Eng. 11, 1–22. Tidwell, J.H., Coyle, S., Webel, C., Evans, J., 1999. Effects and interactions of stocking density and added substrate on production and population structure of freshwater prawns Macrobrachiurn rosenbergii. J. World Aquac. Soc. 30, 174–179. Turnbull, J., Bell, A., Adams, C., Bron, J., Huntingford, F., 2005. Stocking density and welfare of cage farmed Atlantic salmon: application of a multivariate analysis. Aquaculture 243, 121–132. Warren-Myers, F., Swearer, S.E., Francis, D.S., Turchini, G.M., Dempster, T., 2019a. Harvest method does not affect survival and condition during gonad enhancement of an overabundant sea urchin. Aquacult. Environ. Interact. 11, 143–148. Warren-Myers, F., Swearer, S.E., Francis, D.S., Turchini, G.M., Dempster, T., 2019b. Solving key industry bottlenecks for sea urchin roe enhancement. Final Rep. AgriFutures Project No. 010410. https://www.agrifutures.com.au/related-projects/ solving-key-industry-bottlenecks-for-sea-urchin-roe-enhancement/. Worthington, D.G., Blount, C., 2003. Research to Develop and Manage the Sea Urchin Fisheries if NSW and Eastern Victoria. Final Report to the Fisheries Research and Development Corporation Project No. 1999/128. NSW Fisheries Final Report Series No. 56. ISSN 1442-0147. 182pp. Xia, B., Ren, Y., Wang, J., Sun, Y., Zhang, Z., 2017. Effects of feeding frequency and density on growth, energy budget and physiological performance of sea cucumber Apostichopus japonicus (Selenka). Aquaculture 466, 26–32.
Stocking density and rearing environment can affect urchin external condition, GI and gonad grade during onshore roe enhancement aquaculture. Determining the appropriate stocking density to achieve the optimal production per unit area will be a choice of whether to culture urchins in a group reared low-density environment or in an individually reared high-density environment. Relative densities were low in this study and results may be different at higher densities in commercial settings. Urchins used in this trial were cultured in a highwater exchange aquaculture system with water temperatures ranging between 12 and 18 °C. The effects of density or rearing environment may be different if roe enhancement is undertaken at temperatures higher than 18 °C for H. erythrogramma (e.g. Pert et al., 2018), or if water exchange rates are greatly reduced (Motnikar et al., 1998; Mos et al., 2016) or increased. Further research is required to determine whether the effects of density and rearing environment observed in this study will occur during roe enhancement at commercial scale. In addition, a cost benefit analysis of different rearing environments and gonad production per unit area at a commercial scale would enable a better fiscal understanding of what is the most appropriate aquaculture system for sea urchin roe enhancement. Acknowledgements This work was supported by AgriFutures project # PRJ-010410 “Solving key industry bottlenecks for sea urchin roe enhancement” and industry-partner AquaTrophic Pty Ltd. We thank Luke Barrett, Dean Chamberlin, Ben Cleveland and Rebecca Morris for assistance with urchin collection. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.aquaculture.2019.734591. References Andrew, N.L., 1986. The interaction between diet and density in influencing reproductive output in the echinoid Evechinus chloroticus (Val.). J. Exp. Mar. Biol. Ecol. 97, 63–79. Canario, A.V., Condeca, J., Power, D.M., Ingleton, P.M., 1998. The effect of stocking density on growth in the gilthead sea‐bream, Sparus aurata (L.). Aquacult. Res. 29, 177–181. Daggett, T.L., Pearce, C.M., Robinson, S.M., 2006. A comparison of three land‐based containment systems for use in culturing green sea urchins, Strongylocentrotus droebachiensis (Müller) (Echinodermata: Echinoidea). Aquacult. Res. 37, 339–350. Ellis, T., North, B., Scott, A.P., Bromage, N.R., Porter, M., Gadd, D., 2002. The relationships between stocking density and welfare in farmed rainbow trout. J. Fish Biol. 61, 493–531. El‐Sayed, A.F.M., 2002. Effects of stocking density and feeding levels on growth and feed efficiency of Nile tilapia (Oreochromis niloticus L.) fry. Aquacult. Res. 33, 621–626. Evans, J.P., Marshall, D.J., 2005. Male-by-female interactions influence fertilization success and mediate the benefits of polyandry in the sea urchin Heliocidaris erythrogramma. Evolution 59, 106–112. Grisolía, J.M., López, F., de Dios Ortúzar, J., 2012. Sea urchin: from plague to market opportunity. Food Qual. Prefer. 25, 46–56. Huchette, S.M.H., Koh, C.S., Day, R.W., 2003. Growth of juvenile blacklip abalone (Haliotis rubra) in aquaculture tanks: effects of density and ammonia. Aquaculture 219, 457–470. James, P.J., 2006. A comparison of roe enhancement of the sea urchin Evechinus chloroticus in sea-based and land-based cages. Aquaculture 253, 290–300. James, P., 2013. The effect of increased water movement on the roe enhancement of adult sea urchins, Evechinus chloroticus. Aquacult. Res. 44, 1639–1642. James, P., Siikavuopio, S.I., 2015. The effects of tank system, water velocity and water movement on survival, somatic and gonad growth of juvenile and adult green sea urchin, Strongylocentrotus droebachiensis. Aquacult. Res. 46, 1501–1509. James, P.J., Heath, P., Unwin, M.J., 2007. The effects of season, temperature and initial gonad condition on roe enhancement of the sea urchin Evechinus chloroticus. Aquaculture 270, 115–131.
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Stocking density and rearing environment affect external condition, gonad quantity and gonad grade in onshore sea urchin roe enhancement aquaculture
T
Fletcher Warren-Myersa,b,∗, Stephen E. Swearerb, Kathy Overtona, Tim Dempstera a b
Sustainable Aquaculture Laboratory – Temperate and Tropical (SALTT), School of BioSciences, University of Melbourne, Victoria, 3010, Australia National Centre for Coasts and Climate (NCCC) and School of BioSciences, University of Melbourne, Victoria, 3010, Australia
ARTICLE INFO
ABSTRACT
Keywords: Barrens Culture conditions Heliocidaris erythrogramma Intraspecific competition Temperate urchin
Finding the ideal density to optimise growth, health and welfare of aquaculture species reared in cage or tank environments allows farmers to produce the best quality product per unit area. Appropriate stocking densities are well known for most major aquaculture species, but limited information exists for the developing sea urchin aquaculture industry. The temperate purple sea urchin Heliocidaris erythrogramma is a prime candidate for aquaculture, yet how stocking density and/or the rearing environment influence urchin health and roe production remain unknown. Here, we tested whether stocking density (low vs. high) and rearing environment (individual vs. group) influences urchin external condition, gonad index (GI) and roe grade in H. erythrogramma after 12-weeks of roe enhancement during autumn-winter (April–June) and winter-spring (July–September) periods. Across rearing environments, high density reduced the proportion of urchins with healthy external condition by 26–30% during autumn-winter and 7–20% in winter-spring compared to low density. During the autumn-winter period, while higher density resulted in lower GIs in the group reared treatments, higher density did not affect the GI of individually reared urchins. Rearing environment affected gonad grade in the autumnwinter period, with a higher proportion of quality (B-grade) roe produced in individually reared treatments compared to group reared treatments. Our results highlight the importance of stocking density and rearing environment for onshore urchin roe enhancement. Maximising production will require a trade-off between investing in greater infrastructure cost to individually rear urchins at high density to ensure higher GIs versus investing in less expensive infrastructure, but group rearing urchins at lower densities to ensure high GIs after enhancement.
1. Introduction Stocking density is a critical production parameter for aquaculture species, as density can influence an individual's growth rate, health and welfare (Stien et al., 2013). Stocking density affects both highly mobile cultured species (e.g. Atlantic salmon Salmo salar; Turnbull et al., 2005; rainbow trout Oncorhynchus mykiss; Ellis et al., 2002; Nile tilapia Oreochromis niloticus L.; El-Sayed, 2002; gilthead sea-bream, Sparus aurata L.; Canario et al., 1998) and less mobile benthic species, (e.g. blacklip abalone Haliotis rubra; Huchette et al., 2003; prawns Macrobrachiurn rosenbergii; Tidwell et al., 1999; sea cucumbers Apostichous japonicus; Li and Li, 2010; Xia et al., 2017). Density can affect growth either directly through competition for space or food, or indirectly through the buildup of excretory products (Huchette et al., 2003). Some species thrive
when held in high density conditions (e.g. Arctic charr Salvelinus alpinus; Jørgensen et al., 1993) while other species experience reduced growth and/or have higher mortality rates (e.g. red tilapia Oreochromis sp.; Suresh and Lin, 1992). For species new to culture, such as sea urchins undergoing roe enhancement, relatively little is known regarding optimal stocking practices. Sea urchins can occur in densities as high as 80–120 urchins m−2 (Grisolia et al., 2012; Kriegisch et al., 2016) on rocky reefs when they become overabundant and form urchin barrens. When collected from barrens, their gonads are often unmarketable due to very low gonad indices (GIs) (Pert et al., 2018). Low GIs in urchins from barrens are likely due to limited food availability, as urchin densities ∼8 m−2 are enough to limit algal food availability on barrens due to overgrazing (Kriegisch et al., 2016). Roe quality in the temperate urchin Evechinus
∗ Corresponding author. Sustainable Aquaculture Laboratory – Temperate and Tropical (SALTT), School of BioSciences, University of Melbourne, Victoria, 3010, Australia. E-mail address: [email protected] (F. Warren-Myers).
https://doi.org/10.1016/j.aquaculture.2019.734591 Received 28 June 2019; Received in revised form 9 October 2019; Accepted 9 October 2019 Available online 15 October 2019 0044-8486/ © 2019 Elsevier B.V. All rights reserved.
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cool box−1) while in transit to the land-based aquaculture facility in Queenscliff, Victoria. On arrival at the facility, urchins were haphazardly selected from cool boxes and placed into 9 L trays (Internal width 0.26 m, length 0.35 m, height 0.10 m; total surface area 0.213 m2). Each tray sat immersed in the top half of a full 18 L holding tank with either 5 or 10 urchins tray−1 (low or high density), individually separated from each other (individually reared) or allowed to interact with each other (group reared) in the tray (Fig. 1). Each tank set up (18 L holding tank + 9 L tray) received ambient seawater via a 19 mm tap at a continuous flow rate of 0.5–1 L min−1. Each 9 L tray was fitted with a grilled mesh floor (4 mm2 hole size) to allow faecal matter to pass through to a sediment trap at the bottom of each 18 L holding tank. Ambient seawater temperature in the onshore facility progressively dropped from 18.1 to 13.6 °C during the autumn-winter period and remained between 12.8 and 13.5 °C for the winter-spring period. Urchin test diameters ranged from 47.1 ± 0.6 to 48.7 ± 0.6 (mean ± SE) across treatments for the autumn-winter enhancement and 43.2 ± 0.2 to 44.0 ± 0.3 (mean ± SE) across treatments for the winter-spring enhancement (Table 1).
chloroticus appears to depend more on the availability of feed rather than stocking density (Andrew, 1986; James, 2006). However, this does not hold true for all species as in the cool water urchin Strongylocentrotus droebachiensis, increased stocking density did not alter feed intake but negatively affected gonad growth (Siikavuopio et al., 2007). The negative effects of increasing stocking density appear more pronounced for warmer water species of urchins. Among tropical species, stocking densities of 40–180 urchins m−2 (Lytechinus variegatus, Richardson et al., 2011) and 43 to 129 urchins m−2 (Tripneustes gratilla, Mos et al., 2015) have been tested in laboratory experiments. For both species, high stocking density produced negative outcomes. L. variegatus stocked at 180 urchins m−2 resulted in approximately 18% cannibalism, whilst there was no effect at 40 urchins m−2 (Richardson et al., 2011). For T. gratilla, growth rates and relative spine lengths all decreased in high density culture (Mos et al., 2015). The purple sea urchin (Heliocidaris erythrogramma) is an endemic species that inhabits temperate coastal waters in southern Australia (Evans and Marshall, 2005; Pert et al., 2018). In Port Phillip Bay, Victoria, there is an estimated 4300 tons of purple sea urchins, the majority of which reside in urchin barrens at densities ranging from 20 to 120 urchins m−2 (Worthington and Blount, 2003; Johnson et al., 2015; Kriegisch et al., 2016). Local commercial fisheries rarely harvest sea urchins from the barrens due to poor quality roe, which makes them unfit for sale (Sanderson et al., 1996; Pert et al., 2018). However, recent studies have demonstrated that the roe quality of purple sea urchins in barrens can be improved via roe enhancement aquaculture when using high quality feed and optimal environmental conditions (Pert et al., 2018). Previous research on enhancing the roe of purple sea urchins harvested from barrens has primarily focused on feed quality (Pert et al., 2018) and harvesting methods (Warren-Myers et al., 2019a) and used a single stocking density of 10–12 adult urchins per 18–20 L tank. However, optimal stocking density remains unknown as does whether culturing urchins individually or in groups, most benefits roe enhancement. Here, we used two rearing densities, 5 adult urchins per 9 L tank and 10 adult urchins 18 L tank, and cultured urchins individually or in groups, to test if external condition, gonad indices, or gonad grade of urchins collected from barrens habitats were affected. A better understanding of the appropriate rearing environment and optimal stocking density will help fast track the commercial development of roe enhancement aquaculture for H. erythrogramma harvested from urchin barrens.
2.3. Gonad enhancement Urchins were roe enhanced with a pelleted feed that contained optimal levels of lipid and protein for gonad enhancing Heliocidaris erythrogramma (Warren-Myers et al., 2019b) supplied by the Nutrition and Seafood Laboratory (NuSea.Lab), Warrnambool, Victoria, Australia. Feeding (0.6 g urchin−1) occurred three times week−1 for 12 weeks. Faecal matter was siphoned from the settlement area of the 18 L tanks weekly. At 12 weeks post-collection, gonads were dissected, and a gonad index (GI) was estimated using the formula:
% Gonad index =
Urchin gonad wet weight × 100 Urchin total wet weight
2.4. External condition Urchin external condition was assessed by visual assessment and urchins were classified into three classes at the end of the 12-week roe enhancement period as per Warren-Myers et al. (2019a). Classifications were healthy (no spine loss), average (<25% spine loss), or poor (>25% spine loss or with scarring or blemish marks).
2. Methods
2.5. Gonad grade
2.1. Tank environment
Gonads were graded to assess the commercial quality of H. erythrogramma roe based on colour, texture and firmness by following the method and grading table outlined in Pert et al. (2018). Gonad colour was separated into 3 colour categories: (1) bright yellow and orange; (2) pale yellow and orange or dark yellow and orange; and (3) black, brown or grey. Gonads were then separated into 3 texture categories: (1) fine (<1 mm granulation); (2) medium (between 1 and 2 mm granulation); and (3) coarse (>2 mm granulation). Finally, gonads were deemed either (1) firm or (2) not firm based on whether they did or did not hold together during dissection. The grading table (Table 2) was then used to categorise gonads into A, B, C or D grade, were A grade is considered premium quality, B grade high quality, C grade mediocre quality and D grade unacceptable commercial quality.
Urchin density (low or high) and rearing environment (individual or group) (Fig. 1) were manipulated during two separate roe enhancement periods (autumn-winter from April–June and winter-spring from July–September), to assess their effects on external health, gonad production, and gonad grade. The two roe enhancement periods were chosen to align with the recovery, growing and pre-mature stages of the reproduction cycle of H. erythrogramma which typically extends from March to October each year (Pert et al., 2018). 2.2. Collection and tank setup Divers collected urchins for the autumn-winter period on the 4th April and for the winter-spring period on the 10th July 2018. Urchins were collected using a hook and catch bag method (Warren-Myers et al., 2019a) from a rocky urchin barren in the northern part of Port Philip Bay (PPB) in Victoria (PPB; 37.87° S, 144.85° E), approximately 0.5 km offshore. PPB sea surface temperature at the time of each collection date was approximately 18 °C and 13 °C (respectively). Urchins were held in 33 L cool boxes, aerated with pure O2 (0.5–1 L min−1 of O2
2.6. Statistical analysis Mean proportion of healthy urchins, mean gonad indices (%GI) and mean roe grade proportions for each roe enhancement period were analysed using a series of two-way ANOVAs (factor 1, rearing environment = individual vs. group reared; factor 2, density = low vs. high). Untransformed data were used for analysis as raw data met 2
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Fig. 1. The four treatment types. For group reared treatments, low density = 5 urchins. tray−1 and high density = 10 urchins. tray−1. For individually reared treatments, at low density 5 of 6 cells contained urchins and for high density, 10 of 12 cells contained urchins. There were 6 replicate tanks per treatment. Table 1 Rearing environment, stocking density and external condition at harvest for roe enhancement during the autumn-winter and winter-spring. Urchin survival to harvest was 100% for all treatments. Treatment
Replicate
Rearing (I = Individual, G = Group) Density (L = Low, H = High) Autum-Winter IL IH GL GH Winter-Spring IL IH GL GH
Count
Diameters
Stocking density a
Final external conditon
Urchins
Test
Test + spine
Urchins
Urchins
Percentage cover
(n)
(per tray)
(mm ± SE)
(mm ± SE)
m−2
L−1
(% ± SE)
6 6 6 6
5 10 5 10
47.6 ± 0.8 47.1 ± 0.6 48.7 ± 0.6 47.3 ± 0.2
87.2 ± 1.0 86.4 ± 0.9 88.6 ± 1.2 86.7 ± 0.2
23.5 47 23.5 47
0.6 1.1 0.6 1.1
14.0 ± 0.3 27.5 ± 0.5 14.5 ± 0.4 27.8 ± 0.2
100 ± 0 82 ± 18 100 ± 0 70 ± 8
0 16 ± 13 0 22 ± 5
0 2±2 0 8±5
6 6 6 6
5 10 5 10
44.0 ± 0.3 43.5 ± 0.4 43.8 ± 0.3 43.2 ± 0.2
82.9 ± 0.6 81.8 ± 0.4 83.1 ± 0.9 81.0 ± 0.5
23.5 47 23.5 47
0.6 1.1 0.6 1.1
12.7 ± 0.5 24.7 ± 0.6 12.7 ± 0.7 24.2 ± 0.7
97 ± 4 93 ± 4 97 ± 4 80 ± 8
3±3 7±3 3±3 10 ± 4
0 0 0 10 ± 4
Healthy
Average
Poor
% ± SE)
a
Test + spine diameters are estimated from test diameters using the linear model y = 1.3504x + 22.847, R2 = 0.782 which was fitted to test and test + spine diameter data measured from a subsample of 30 urchins.
ANOVA assumptions of homogeneity of variances and normality. All values are reported as the mean ± 1 Standard Error (SE). Urchin density is reported (Table 1.) as number urchins per m2 of internal surface area of tank, number urchins per L, and estimated percentage cover of internal tank surface area using maximum urchin diameter (urchin test + spines) (James et al., 2017) to allow for comparisons between this study and other research.
healthy external condition range from 70 to 100% during the autumnwinter period and 80–97% during the winter-spring roe enhancement period (Table 1). During the autumn-winter, there was no interaction between density and rearing environment (F1, 23 = 0.1, p = 0.5) and no effect of rearing environment (F1, 23 = 0.1, p = 0.5), but there was an effect of density (F1, 23 = 9.8, p = 0.005). At the end of the enhancement, both low density treatments had 100% of urchins in healthy condition, whilst urchins cultured at high density averaged 70 ± 8% for group reared urchins and 82 ± 18% for individually reared urchins (Table 1). The same pattern was observed during the winter-spring enhancement period, with no interaction between density and rearing environment (F1, 23 = 2.6, p = 0.1) or an effect of rearing environment (F1, 23 = 2.6, p = 0.1), but an effect of density (F1, 23 = 6.1, p = 0.023).
3. Results 3.1. External condition Across treatments, the proportion of urchins classed as having 3
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Table 2 The grading system used to assess the commercial quality of H. erythrogramma roe based on colour, texture and firmness. A grade is premium quality, B grade is high quality, C grade is mediocre quality, while D grade is of unacceptable commercial quality. ‘F’ is a firm gonad, while ‘NF’ is not firm (Pert et al., 2018).
Table 3 ANOVA summary of effect of rearing environment and density on gonad grade distributions during autumn-winter (April–June) and winter-spring (July–September) roe enhancement periods. *indicates significant effect at p < 0.05.
Colour
Texture
Firmness
Grade
Autumn - winter
Winter - spring
Bright orange or yellow
Fine
F NF F NF F NF F NF F NF F NF F NF F NF F NF
A B A B B B B C B C C C D D D D D D
F1,23
P
F1,23
p
0.756 1.345 4.118
0.395 0.260 0.056
0.179 0.738 0.000
0.676 0.400 0.990
3.034 0.150 6.330
0.097 0.703 0.021*
0.190 4.198 0.001
0.668 0.054 0.980
0.016 0.016 1.958
0.900 0.900 0.177
0.054 0.054 0.054
0.819 0.819 0.819
1.984 2.770 0.160
0.174 0.112 0.899
0.044 2.792 0.044
0.837 0.110 0.837
Medium Coarse Pale/dark orange or yellow
Fine Medium Coarse
Black, brown, grey
Fine Medium Coarse
A grade Density*Rearing environment Density Rearing environment B grade Density*Rearing environment Density Rearing environment C grade Density*Rearing environment Density Rearing environment D grade Density*Rearing environment Density Rearing environment
Both low density treatments averaged 97 ± 4% of urchins in healthy condition after 12 weeks of roe enhancement, whilst urchins cultured at high density averaged 80 ± 8% for group reared urchins and 93 ± 4% for individually reared urchins (Table 1).
(Table 3, Fig. 3). Individually reared treatments produced an average of 68 ± 5% B grade roe, whereas group reared treatments produced 46 ± 8% B grade roe. There were no other significant effects of rearing environment or density on gonad grade distribution (Table 3).
3.2. Gonad indices
4. Discussion
After roe enhancement, GIs ranged from 13 to 17% during the autumn-winter and 14–16% during the winter-spring periods across treatments (Fig. 2). During autumn-winter, there was an interaction between density and rearing environment (F1, 23 = 8, p = 0.013) as rearing environment influenced the effects of density on final GIs. Lowdensity group reared urchins produced higher GIs than high-density group reared urchins, but there was no difference in GI between low and high densities in the individually reared treatments and the lowdensity group reared treatment (Fig. 2). During the winter-spring enhancement period, there was no interaction between density and rearing environment (F1, 23 = 1.0, p = 0.3) and no effect of density (F1, 23 = 4.0, p = 0.1) or rearing environment (F1, 23 = 0.6, p = 0.4, Fig. 2).
Optimal culturing conditions for Heliocidaris erythrogramma were dependent on both stocking density and whether urchins were reared individually or in groups. Here we demonstrated that higher stocking densities negatively affect the external condition of urchins regardless of how they were reared. Higher density also reduced final GIs after roe enhancement, but this effect can be mitigated if urchins are isolated from one another in the rearing environment rather than reared in groups. 4.1. External condition The external condition of H. erythogramma at harvest was best under low stocking densities for both roe enhancement periods. The external appearance of an urchin is important for marketability if they are sold as a live export product, as roe quality is directly linked with external appearance in certain species of urchin (Mos and Dworjanyn, 2019). The occurrence of external injuries is also a known indicator of poor gonad growth (Siikavuopio et al., 2007). The greater occurrence of external injuries (spine loss or damage) with higher stocking density is most likely due to intraspecific competition for space which may cause increased animal stress. For example, the urchin Lytechinus variegatus attacks and cannibalises other urchins when held in high density (180 urchins m−2) (Richardson et al., 2011). Decreased spine lengths can also occur in high density culture (Tripnuestes gratilla: Mos et al., 2015). Furthermore, Siikavuopio et al. (2007) found that mortality and the occurrence of injuries dramatically increased when stocking densities exceeded 86 urchins m−2 for the adult (>70 g) green sea urchins (Strongylocentrotu droebachiensis). Rearing environment had no overall effect on external condition. While isolating urchins stopped any direct effects of intraspecific competition resulting in injuries or spine loss, any benefits of improving external health by rearing urchins individually may have been offset by the confined space restricting movement, hence causing spine damage or loss. Siikavuopio et al. (2007) used individually compartmentalised units (12 × 12 × 18 cm) for urchins in a density trial on S.
3.3. Gonad grade Rearing environment influenced the proportion of high-quality (Bgrade) gonads during the autumn-winter roe enhancement period
Fig. 2. Gonad indices (GI%) measured in the purple sea urchin Heliocidaris erythrogramma after 12 weeks of roe enhancement during the autumn-winter (April–June) and winter-spring (July–September) periods. Low density = 5 urchins. tray−1, high density = 10 urchins. tray−1. Error bars ±1 Standard Error. 4
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Fig. 3. Gonad grade distribution measured in the purple sea urchin Heliocidaris erythrogramma after 12 weeks of roe enhancement during the autumn-winter (April–June) and winter-spring (July–September) periods. Low density = 5 urchins. tray−1, high density = 10 urchins. tray−1. Group or individually refers to rearing environment in tray. N = 6 tanks per treatment. Gonads were graded into A, B, C, or D grade following Pert et al. (2018) methods. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
droebachiensis and reported no external injuries when urchins were held individually. However, their individual compartments (0.0026 m3 for each urchin) were approximately 1.7–3.4 times larger in total volume than the individual compartments in our individual rearing treatments (Low density - 0.0015 m3, High density - 0.00076 m3, for each urchin, respectively) meaning any effect on external condition due to restricted movement in a confined space may not have arisen in their study. Determining the optimal minimum compartment size to individually house urchins to ensure healthy external condition will require further investigation as compartment size may also be a function of urchin size.
and all urchins were sourced from the one collection site in PPB. Hence, why roe grade was lower in individually reared treatments is unclear. 4.4. Autumn-winter vs. winter-spring enhancement Final GIs differed across treatments during the autumn-winter enhancement, but not during winter-spring enhancement. This may have been due to differences in seasonal temperature profiles (James et al., 2007) during each period, more so than the difference in average size of urchins used (Pearce et al., 2004) for each period. Ambient seawater temperature in the autumn-winter period dropped from 18.1 to 13.6 °C but was cooler and more stable between 12.8 and 13.5 °C for the winterspring period. Average urchin test size was ∼10% larger during the autumn-winter compared to the winter spring period (Table 1). Urchins during the autumn-winter period may have been more active due to experiencing higher water temperatures (Percy, 1973), and coupled with a slightly larger average test size, this may have led to greater competitive interactions. This may explain the differences observed in GIs amongst treatments during the autumn-winter enhancement period (autumn-spring GIs 13.2–17.4%; winter-spring GIs 14.3–16.2%) but not the winter-spring period.
4.2. Gonad indices Gonad indices (GIs) exceeded the recommended marketable minimum of 10–12.5% (Lawrence et al., 2011; Pert et al., 2018) across all treatments and ranged from 13.2 to 17.4% during autumn-winter and 14.3–16.2% in winter-spring. Final GIs differed between the high and low density for group reared urchins, but not individually reared urchins, only during the autumn-winter enhancement period. An interaction between rearing environment and urchin density during the autumn-winter enhancement highlighted that rearing urchins individually during roe enhancement reduced the difference in final GI between the low density (5 per tray) and high density (10 per tray) treatments. Social interaction can increase in sea urchins with increasing density (Qi et al., 2016), hence eliminating interactions and/or competition between urchins through rearing environment may have allowed a greater amount of metabolic energy to be allocated to gonad growth. The results indicate that the decision of whether to cultivate urchins individually or in groups will be a trade-off between desired GI per urchin and number of urchins per unit area cultured. Here, while the low-density group reared treatment produced the overall highest GIs for both enhancement periods (Fig. 2), in terms of production per unit area, the high density individually reared urchins would produce approximately 1.9 times more gonad biomass than low-density group reared urchins.
4.5. Tank design Tank design, water exchange, and the way urchins a housed are important factors to consider when designing an urchin aquaculture system (e.g. Motnikar et al., 1998; Daggett et al., 2006; Siikavuopio et al., 2007; James, 2013; James and Siikavuopio, 2015). Here, urchins were housed in shallow 10 cm deep trays, that sat inside holding tanks (total depth ∼ 20 cm) with a high-water exchange rate (2–4 times per hour). No mortalities occurred during enhancement, however scaling up to a commercial level may have potential issues, as tank size (e.g. large race ways vs. small race ways Daggett et al., 2006) or lower water exchange rates may result in higher mortalities (Motnikar et al., 1998) or poorer growth rates (Mos et al., 2016). Caging urchins individually in shallow tanks limits intraspecific competition, which allows for increased urchin density while still maintaining marketable gonad quantities. However, extra physical structures in tanks used to keep urchins separated could cause issues with cage cleaning, or the potential for areas of larger shallow tanks to have poorer water exchange. Implementing the use of bucket tippers, in shallow race way tanks could be a way to increase water movement and assist with cleaning individual cage structures (e.g. James and Siikavuopio, 2015). This would allow for urchins to be housed individually in cages suspended in shallow raceways, while still maintaining suitable water circulation.
4.3. Gonad grade A greater proportion of B-grade roe was produced in the individually reared treatments (58–76%) compared to the group reared treatments (40–52%) during the autumn-winter enhancement period. More B grade roe in the individually reared treatments appeared to offset by a combination of less A or C grade roe during the autumnwinter, but this pattern did not hold for the winter-spring enhancement period (Fig. 3). In H. erythrogramma, feed type influences roe grade (Senaratna et al., 2005; Pert et al., 2018) as does the collection site (Pert et al., 2018). However, we used only one feed type for all treatments 5
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5. Conclusion
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Stocking density and rearing environment can affect urchin external condition, GI and gonad grade during onshore roe enhancement aquaculture. Determining the appropriate stocking density to achieve the optimal production per unit area will be a choice of whether to culture urchins in a group reared low-density environment or in an individually reared high-density environment. Relative densities were low in this study and results may be different at higher densities in commercial settings. Urchins used in this trial were cultured in a highwater exchange aquaculture system with water temperatures ranging between 12 and 18 °C. The effects of density or rearing environment may be different if roe enhancement is undertaken at temperatures higher than 18 °C for H. erythrogramma (e.g. Pert et al., 2018), or if water exchange rates are greatly reduced (Motnikar et al., 1998; Mos et al., 2016) or increased. Further research is required to determine whether the effects of density and rearing environment observed in this study will occur during roe enhancement at commercial scale. In addition, a cost benefit analysis of different rearing environments and gonad production per unit area at a commercial scale would enable a better fiscal understanding of what is the most appropriate aquaculture system for sea urchin roe enhancement. Acknowledgements This work was supported by AgriFutures project # PRJ-010410 “Solving key industry bottlenecks for sea urchin roe enhancement” and industry-partner AquaTrophic Pty Ltd. We thank Luke Barrett, Dean Chamberlin, Ben Cleveland and Rebecca Morris for assistance with urchin collection. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.aquaculture.2019.734591. References Andrew, N.L., 1986. The interaction between diet and density in influencing reproductive output in the echinoid Evechinus chloroticus (Val.). J. Exp. Mar. Biol. Ecol. 97, 63–79. Canario, A.V., Condeca, J., Power, D.M., Ingleton, P.M., 1998. The effect of stocking density on growth in the gilthead sea‐bream, Sparus aurata (L.). Aquacult. Res. 29, 177–181. Daggett, T.L., Pearce, C.M., Robinson, S.M., 2006. A comparison of three land‐based containment systems for use in culturing green sea urchins, Strongylocentrotus droebachiensis (Müller) (Echinodermata: Echinoidea). Aquacult. Res. 37, 339–350. Ellis, T., North, B., Scott, A.P., Bromage, N.R., Porter, M., Gadd, D., 2002. The relationships between stocking density and welfare in farmed rainbow trout. J. Fish Biol. 61, 493–531. El‐Sayed, A.F.M., 2002. Effects of stocking density and feeding levels on growth and feed efficiency of Nile tilapia (Oreochromis niloticus L.) fry. Aquacult. Res. 33, 621–626. Evans, J.P., Marshall, D.J., 2005. Male-by-female interactions influence fertilization success and mediate the benefits of polyandry in the sea urchin Heliocidaris erythrogramma. Evolution 59, 106–112. Grisolía, J.M., López, F., de Dios Ortúzar, J., 2012. Sea urchin: from plague to market opportunity. Food Qual. Prefer. 25, 46–56. Huchette, S.M.H., Koh, C.S., Day, R.W., 2003. Growth of juvenile blacklip abalone (Haliotis rubra) in aquaculture tanks: effects of density and ammonia. Aquaculture 219, 457–470. James, P.J., 2006. A comparison of roe enhancement of the sea urchin Evechinus chloroticus in sea-based and land-based cages. Aquaculture 253, 290–300. James, P., 2013. The effect of increased water movement on the roe enhancement of adult sea urchins, Evechinus chloroticus. Aquacult. Res. 44, 1639–1642. James, P., Siikavuopio, S.I., 2015. The effects of tank system, water velocity and water movement on survival, somatic and gonad growth of juvenile and adult green sea urchin, Strongylocentrotus droebachiensis. Aquacult. Res. 46, 1501–1509. James, P.J., Heath, P., Unwin, M.J., 2007. The effects of season, temperature and initial gonad condition on roe enhancement of the sea urchin Evechinus chloroticus. Aquaculture 270, 115–131.
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