Size matters: How single-species management can contribute to ecosystem-based fisheries management

In this study we show how substantial gains towards the goals of ecosystem-based fisheries management (EBFM) can be achieved by different single-species management. We show that fishing has much less impact on stocks if fish are caught after they have reached the size (L_opt) where growth rate and cohort biomass are maximum. To demonstrate our point we compare the impact of three fishing scenarios on 9 stocks from the North Sea and the Baltic. Scenario (1) is the current fishing regime, scenario (2) is a new management regime proposed by the European Commission, aiming for maximum sustainable yield obtained from all stocks, and scenario (3) is set so that it achieves the same yield as scenario (2), albeit with fishing on sizes beyond L_opt. Results show that scenarios (2) and (3) are significant improvements compared to current fishing practice. However, scenario (3) consistently shows least impact on the stocks, with seven-fold higher biomass of demersal fishes and an age structure similar to an unfished stock. This allows juveniles and adults to better fulfil their ecological roles, a major step towards the goals of ecosystem-based fisheries management. We give examples where scenario (3) is practiced in successful fisheries. We present a new interpretation of the relative yield per recruit isopleth diagram with indication of a new target area for fisheries operating within the context of EBFM. We present a new expression of the relative biomass per recruit isopleth diagram, which supports our analysis. We conclude that size matters for precautionary and ecosystem-based fisheries management and present a list of additional advantages associated with fishing at L_opt.     

Influence of guanidination on apparent ileal digestibility of amino acids in pigs fed diets with soybean meal, rapeseed meal or peas as a protein source

The objective of this study was to determine the effect of guanidination, the conversion of dietary lysine to homoarginine, on the apparent ileal digestibilities (AID) of amino acids (AA) from 3 protein sources in diets for pigs. Six barrows, initial body weight of 18 ± 1.5 kg, fitted with a simple T-cannula at the distal ileum, were fed one of three corn starch-based diets that contained 18% crude protein (CP) from soybean meal, rapeseed meal or peas for an experimental period of 10 d according to a repeated 3 × 3 Latin square design. On d 9 of each experimental period, the pigs were fed one meal of the guanidinated test meals, which corresponded to their respective diets. In the test meals, 50% of the protein source in each diet was replaced, on an equal weight basis, with a guanidinated source. Guanidination increased the content of CP by 190, 116 and 28 g kg^− 1 for soybean meal, rapeseed meal and peas, respectively. With the exception of lysine, the AID of CP and AA were 2.2 to 8.2 percentage units (pu) higher in the test meal than in the soybean meal diet. Other than arginine and isoleucine, the differences were significant (P < 0.05) for all indispensable AA. With the exception of lysine, the AID of CP and AA were 3.8 to 11.9 pu higher in the test meal than in the diet with peas. The differences were significant (P < 0.05) for all indispensable AA. Other than lysine, glycine and proline, the AID of CP and AA were slightly higher, 0.1 to 4.3 pu, in the test meal than in the rapeseed meal diet. Of the indispensable AA, the differences were significant (P < 0.05) for arginine, leucine and phenylalanine. The AID of lysine was 10.9 pu lower (P < 0.05) in the test meal than in the rapeseed meal diet, 61.5 vs. 72.4%, indicating that guanidination of rapeseed meal was not uniform and random. The increase in the AID of CP and AA after guanidination may result from solubilisation of protein and degradation of antinutritional factors such as fiber, trypsin inhibitors and tannins.     

Effective stress and shear strength parameters of unsaturated soils as affected by compaction and subsequent shearing

Wheeling induced compression and shearing forces are main stresses accounting for soil deformation and changes of hydraulic, gaseous and thermal properties. There are reports about the combined effects of compaction and subsequent shearing on soil hydraulic properties, but their consequences on soil strength properties (i.e., effective stress and shear strength) need to be further analysed. This study investigated the dynamics of soil mechanical properties as affected by pore water pressure ($u_{\mathrm{w}}$) during compaction and shearing. Soil samples from an A-horizon of Gleysols derived from glacial sediment and a Stagnic Luvisol from loess were analysed. The repacked and structured samples were compressed under static and cyclic loading and then sheared at two speeds (0.3 and 2.0 mm min⁻¹) with three loading levels (50, 100, and 200 kPa). During each stress application, the $u_{\mathrm{w}}$, chi factor (χ) and effective stress (${\sigma }^{\prime }$) were measured and calculated. The shear strength ($\tau$), angle of internal friction ($\phi$) and cohesion ($c$), were determined and fitted by the Mohr–Coulomb failure criterion. The results showed that compaction and shearing increased $u_{\mathrm{w}}$ and χ in all homogenized soils while on structured soils this phenomenon only occurred when the applied loading stress exceeded the soil precompression stress. The increased $u_{\mathrm{w}}$ resulted in soil hydraulic and mechanical stresses, which ultimately reduced the ${\sigma }^{\prime }$, especially at −6 kPa initial matric potential. Soils with finer texture, higher loading stresses and faster shear speed normally exhibited more reduced ${\sigma }^{\prime }$ values. The structured soil had higher $\tau$ values with higher $\phi$ and $c$ compared to the homogenized soils. The changes of $u_{\mathrm{w}}$ at high loading stress (i.e., 200 kPa) may overlap the normal pattern of the Mohr–Coulomb failure line that results from the theoretical Mohr envelope. Thus, to minimize the destruction of soil structure and stability induced by wheeling, it is important to consider field water content, traffic loading and wheeling speed.